📅 Bash Dates & Time

Essential date formats for system administration—logs, backups, and reports.

#!/bin/bash
# Date formatting for various purposes

echo "=== Standard Formats ==="
echo "Default:      $(date)"
echo "ISO 8601:     $(date +%Y-%m-%dT%H:%M:%S%z)"
echo "ISO Date:     $(date +%Y-%m-%d)"
echo "ISO Time:     $(date +%H:%M:%S)"
echo "RFC 2822:     $(date -R)"
echo "UTC:          $(date -u '+%Y-%m-%d %H:%M:%S UTC')"

echo ""
echo "=== Regional Formats ==="
echo "US Date:      $(date +%m/%d/%Y)"
echo "US Long:      $(date '+%B %d, %Y')"
echo "EU Date:      $(date +%d/%m/%Y)"
echo "EU Long:      $(date '+%d %B %Y')"

echo ""
echo "=== Filename-Safe Formats ==="
echo "Daily backup:   backup-$(date +%Y-%m-%d).tar.gz"
echo "Timestamped:    log-$(date +%Y%m%d_%H%M%S).txt"
echo "Hourly:         data-$(date +%Y-%m-%d_%H).csv"
echo "Weekly:         report-$(date +%Y-W%W).pdf"
echo "Monthly:        archive-$(date +%Y-%m).zip"

echo ""
echo "=== Log Entry Formats ==="
# Syslog style
echo "$(date '+%b %d %H:%M:%S') $(hostname) myapp[$$]: Message"

# ISO style (recommended)
echo "[$(date '+%Y-%m-%d %H:%M:%S')] INFO: Application started"

# With milliseconds
echo "[$(date '+%Y-%m-%d %H:%M:%S.%3N')] DEBUG: Detailed message"

echo ""
echo "=== Report Headers ==="
echo "Generated: $(date '+%A, %B %e, %Y at %I:%M %p %Z')"
echo "Timestamp: $(date '+%Y-%m-%d %H:%M:%S %Z')"

echo ""
echo "=== Special Values ==="
echo "Epoch:        $(date +%s)"
echo "Nanoseconds:  $(date +%N)"
echo "Day of year:  $(date +%j) / 365"
echo "Week number:  $(date +%W) / 52"
echo "Quarter:      Q$(( ($(date +%-m) - 1) / 3 + 1 ))"

Calculating future and past dates for scheduling, retention, and planning.

#!/bin/bash
# Date arithmetic with -d flag

echo "=== Reference Point ==="
echo "Today: $(date '+%Y-%m-%d %A')"

echo ""
echo "=== Adding Time ==="
echo "Tomorrow:      $(date -d 'tomorrow' '+%Y-%m-%d %A')"
echo "+1 day:        $(date -d '+1 day' '+%Y-%m-%d')"
echo "+3 days:       $(date -d '+3 days' '+%Y-%m-%d')"
echo "+1 week:       $(date -d '+1 week' '+%Y-%m-%d')"
echo "+2 weeks:      $(date -d '+2 weeks' '+%Y-%m-%d')"
echo "+1 month:      $(date -d '+1 month' '+%Y-%m-%d')"
echo "+3 months:     $(date -d '+3 months' '+%Y-%m-%d')"
echo "+1 year:       $(date -d '+1 year' '+%Y-%m-%d')"

echo ""
echo "=== Subtracting Time ==="
echo "Yesterday:     $(date -d 'yesterday' '+%Y-%m-%d %A')"
echo "-1 day:        $(date -d '-1 day' '+%Y-%m-%d')"
echo "-7 days:       $(date -d '-7 days' '+%Y-%m-%d')"
echo "-30 days:      $(date -d '-30 days' '+%Y-%m-%d')"
echo "-3 months:     $(date -d '-3 months' '+%Y-%m-%d')"
echo "-1 year:       $(date -d '-1 year' '+%Y-%m-%d')"

echo ""
echo "=== Time Arithmetic ==="
echo "Now:           $(date '+%H:%M:%S')"
echo "+2 hours:      $(date -d '+2 hours' '+%H:%M:%S')"
echo "-30 minutes:   $(date -d '-30 minutes' '+%H:%M:%S')"
echo "+90 seconds:   $(date -d '+90 seconds' '+%H:%M:%S')"

echo ""
echo "=== Combined Arithmetic ==="
echo "+1 day 3 hours:    $(date -d '+1 day +3 hours' '+%Y-%m-%d %H:%M')"
echo "tomorrow noon:     $(date -d 'tomorrow 12:00' '+%Y-%m-%d %H:%M')"
echo "next monday 9am:   $(date -d 'next monday 09:00' '+%Y-%m-%d %H:%M')"

echo ""
echo "=== From Specific Date ==="
base_date="2025-01-15"
echo "Base: $base_date"
echo "+30 days:      $(date -d "$base_date +30 days" '+%Y-%m-%d')"
echo "+6 months:     $(date -d "$base_date +6 months" '+%Y-%m-%d')"
echo "End of month:  $(date -d "$base_date +1 month -1 day" '+%Y-%m-%d')"

echo ""
echo "=== Practical: Retention Dates ==="
echo "Delete files older than:"
echo "  7 days:   $(date -d '-7 days' '+%Y-%m-%d')"
echo "  30 days:  $(date -d '-30 days' '+%Y-%m-%d')"
echo "  90 days:  $(date -d '-90 days' '+%Y-%m-%d')"
echo "  1 year:   $(date -d '-1 year' '+%Y-%m-%d')"

Converting between date formats and parsing various input strings.

#!/bin/bash
# Date parsing and format conversion

echo "=== Parse Various Input Formats ==="
inputs=(
    "November 18, 2025"
    "Nov 18, 2025"
    "18 Nov 2025"
    "11/18/2025"
    "2025-11-18"
    "2025/11/18"
    "18-Nov-2025"
)

for input in "${inputs[@]}"; do
    parsed=$(date -d "$input" '+%Y-%m-%d' 2>/dev/null)
    if [[ $? -eq 0 ]]; then
        printf "%-20s → %s\n" "$input" "$parsed"
    else
        printf "%-20s → FAILED\n" "$input"
    fi
done

echo ""
echo "=== Parse with Time ==="
echo "'2025-11-18 14:30':     $(date -d '2025-11-18 14:30' '+%Y-%m-%d %H:%M:%S')"
echo "'Nov 18 2:30 PM':       $(date -d 'Nov 18 2:30 PM' '+%Y-%m-%d %H:%M:%S')"
echo "'2025-11-18T14:30:00':  $(date -d '2025-11-18T14:30:00' '+%Y-%m-%d %H:%M:%S')"

echo ""
echo "=== Convert Between Formats ==="

# US to ISO
us_date="11/18/2025"
iso=$(date -d "$us_date" '+%Y-%m-%d')
echo "US to ISO: $us_date → $iso"

# ISO to verbose
iso_date="2025-11-18"
verbose=$(date -d "$iso_date" '+%A, %B %d, %Y')
echo "ISO to verbose: $iso_date → $verbose"

# Short to long
short="Nov 18"
long=$(date -d "$short" '+%B %d, %Y')
echo "Short to long: $short → $long"

echo ""
echo "=== Epoch Conversions ==="
# Current to epoch
now_epoch=$(date +%s)
echo "Now to epoch: $(date '+%Y-%m-%d %H:%M:%S') → $now_epoch"

# Epoch to human
echo "Epoch to human: 1731958245 → $(date -d @1731958245 '+%Y-%m-%d %H:%M:%S')"

# String to epoch
date_str="2025-12-25 00:00:00"
epoch=$(date -d "$date_str" +%s)
echo "String to epoch: $date_str → $epoch"

echo ""
echo "=== Parse Log Timestamps ==="

# Syslog format
syslog="Nov 18 14:30:45"
parsed=$(date -d "$syslog" '+%Y-%m-%d %H:%M:%S')
echo "Syslog: $syslog → $parsed"

# Apache format (need to adjust)
apache="18/Nov/2025:14:30:45"
adjusted=$(echo "$apache" | sed 's/\// /g; s/:/ /')
parsed=$(date -d "$adjusted" '+%Y-%m-%d %H:%M:%S')
echo "Apache: $apache → $parsed"

echo ""
echo "=== Validate Date Input ==="
validate_date() {
    date -d "$1" '+%Y-%m-%d' &>/dev/null
}

test_dates=("2025-11-18" "2025-02-30" "invalid" "13/45/2025")
for d in "${test_dates[@]}"; do
    if validate_date "$d"; then
        echo "'$d' is VALID"
    else
        echo "'$d' is INVALID"
    fi
done

Computing elapsed time, age, and duration between two dates.

#!/bin/bash
# Calculating time differences

echo "=== Days Between Two Dates ==="
date1="2025-01-01"
date2="2025-11-18"

epoch1=$(date -d "$date1" +%s)
epoch2=$(date -d "$date2" +%s)
diff_secs=$((epoch2 - epoch1))
diff_days=$((diff_secs / 86400))

echo "From: $date1"
echo "To:   $date2"
echo "Days: $diff_days"
echo "Weeks: $((diff_days / 7))"
echo "Months (approx): $((diff_days / 30))"

echo ""
echo "=== Time Until Future Date ==="
future="2025-12-25"
now=$(date +%s)
future_epoch=$(date -d "$future" +%s)
until_secs=$((future_epoch - now))
until_days=$((until_secs / 86400))

echo "Days until $future: $until_days"

echo ""
echo "=== Age Calculation ==="
birth_date="1982-06-25"
today=$(date +%s)
birth_epoch=$(date -d "$birth_date" +%s)
age_secs=$((today - birth_epoch))
age_days=$((age_secs / 86400))
age_years=$((age_days / 365))

echo "Birth: $birth_date"
echo "Age: ~$age_years years ($age_days days)"

echo ""
echo "=== Format Duration ==="
format_duration() {
    local secs=$1
    local days=$((secs / 86400))
    local hours=$(( (secs % 86400) / 3600 ))
    local mins=$(( (secs % 3600) / 60 ))
    local s=$((secs % 60))
    
    if ((days > 0)); then
        printf "%dd %dh %dm %ds" $days $hours $mins $s
    elif ((hours > 0)); then
        printf "%dh %dm %ds" $hours $mins $s
    elif ((mins > 0)); then
        printf "%dm %ds" $mins $s
    else
        printf "%ds" $s
    fi
}

echo "Duration examples:"
for secs in 45 3661 86400 90061 604800; do
    printf "%8d seconds = %s\n" $secs "$(format_duration $secs)"
done

echo ""
echo "=== Working Days Between Dates ==="
count_workdays() {
    local start=$1
    local end=$2
    local count=0
    local current=$start
    
    while [[ "$current" < "$end" ]]; do
        local dow=$(date -d "$current" +%u)
        if ((dow < 6)); then  # Mon-Fri
            ((count++))
        fi
        current=$(date -d "$current +1 day" +%Y-%m-%d)
    done
    
    echo $count
}

start="2025-11-01"
end="2025-11-18"
workdays=$(count_workdays "$start" "$end")
echo "Working days from $start to $end: $workdays"

echo ""
echo "=== Practical: SLA Calculation ==="
ticket_opened="2025-11-18 09:00:00"
ticket_closed="2025-11-18 14:30:45"

open_epoch=$(date -d "$ticket_opened" +%s)
close_epoch=$(date -d "$ticket_closed" +%s)
resolution_secs=$((close_epoch - open_epoch))
resolution_mins=$((resolution_secs / 60))
resolution_hours=$(echo "scale=2; $resolution_secs / 3600" | bc)

echo "Opened: $ticket_opened"
echo "Closed: $ticket_closed"
echo "Resolution time: ${resolution_mins} minutes (${resolution_hours} hours)"

Converting between timezones and displaying times for global teams.

#!/bin/bash
# Timezone operations

echo "=== Current Time in Various Zones ==="
echo "Local:      $(date '+%Y-%m-%d %H:%M:%S %Z')"
echo "UTC:        $(TZ=UTC date '+%Y-%m-%d %H:%M:%S %Z')"

echo ""
echo "US Timezones:"
echo "  Eastern:  $(TZ=America/New_York date '+%H:%M %Z')"
echo "  Central:  $(TZ=America/Chicago date '+%H:%M %Z')"
echo "  Mountain: $(TZ=America/Denver date '+%H:%M %Z')"
echo "  Pacific:  $(TZ=America/Los_Angeles date '+%H:%M %Z')"

echo ""
echo "International:"
echo "  London:   $(TZ=Europe/London date '+%H:%M %Z')"
echo "  Paris:    $(TZ=Europe/Paris date '+%H:%M %Z')"
echo "  Tokyo:    $(TZ=Asia/Tokyo date '+%H:%M %Z')"
echo "  Sydney:   $(TZ=Australia/Sydney date '+%H:%M %Z')"

echo ""
echo "=== Convert Specific Time Between Zones ==="
# Meeting at 2 PM Eastern - what time elsewhere?
meeting_time="2025-11-18 14:00:00"
meeting_epoch=$(TZ=America/New_York date -d "$meeting_time" +%s)

echo "Meeting: $meeting_time Eastern"
echo ""
echo "Equivalent times:"
for tz in "America/Los_Angeles:Pacific" "America/Chicago:Central" \
          "UTC:UTC" "Europe/London:London" "Asia/Tokyo:Tokyo"; do
    IFS=':' read -r zone label <<< "$tz"
    converted=$(TZ=$zone date -d @$meeting_epoch '+%H:%M %Z (%Y-%m-%d)')
    printf "  %-10s %s\n" "$label:" "$converted"
done

echo ""
echo "=== Parse Time with Timezone ==="
# Input time is in Pacific, convert to local
pacific_time="2025-11-18 09:00:00"
pacific_epoch=$(TZ=America/Los_Angeles date -d "$pacific_time" +%s)

echo "Source: $pacific_time Pacific"
echo "Local:  $(date -d @$pacific_epoch '+%Y-%m-%d %H:%M:%S %Z')"
echo "UTC:    $(TZ=UTC date -d @$pacific_epoch '+%Y-%m-%d %H:%M:%S %Z')"

echo ""
echo "=== Timezone Offset Information ==="
echo "Current timezone: $(date +%Z)"
echo "UTC offset: $(date +%z)"
echo "Offset with colon: $(date +%:z)"

echo ""
echo "=== Store in UTC, Display Local ==="
# Best practice: store timestamps in UTC
stored_utc="2025-11-18T19:30:45Z"

# Display in various local times
echo "Stored (UTC): $stored_utc"
for tz in "America/New_York" "America/Los_Angeles" "Europe/London"; do
    local_time=$(TZ=$tz date -d "$stored_utc" '+%Y-%m-%d %H:%M:%S %Z')
    printf "  %-20s %s\n" "$tz:" "$local_time"
done

echo ""
echo "=== Practical: Global Team Meeting Planner ==="
echo ""
echo "Finding a good meeting time (9 AM - 5 PM for all):"

# Check 14:00 UTC
utc_hour=14
echo "Testing ${utc_hour}:00 UTC:"

zones=("America/New_York" "America/Los_Angeles" "Europe/London" "Asia/Tokyo")
all_good=true

for tz in "${zones[@]}"; do
    local_hour=$(TZ=$tz date -d "2025-11-18 ${utc_hour}:00 UTC" '+%H')
    local_time=$(TZ=$tz date -d "2025-11-18 ${utc_hour}:00 UTC" '+%H:%M %Z')
    
    if ((10#$local_hour >= 9 && 10#$local_hour < 17)); then
        status="✓"
    else
        status="✗"
        all_good=false
    fi
    
    printf "  %s %-20s %s\n" "$status" "$tz" "$local_time"
done

Finding specific weekdays, first/last of month, and calendar calculations.

#!/bin/bash
# Weekday and calendar operations

echo "=== Current Day Information ==="
echo "Day name: $(date '+%A')"
echo "Day number (Mon=1): $(date '+%u')"
echo "Day number (Sun=0): $(date '+%w')"
echo "Day of month: $(date '+%d')"
echo "Day of year: $(date '+%j')"

echo ""
echo "=== Find Specific Weekdays ==="
echo "Next Monday:    $(date -d 'next monday' '+%Y-%m-%d')"
echo "Next Friday:    $(date -d 'next friday' '+%Y-%m-%d')"
echo "Last Sunday:    $(date -d 'last sunday' '+%Y-%m-%d')"
echo "This Wednesday: $(date -d 'this wednesday' '+%Y-%m-%d')"

echo ""
echo "=== This Week's Days ==="
for day in monday tuesday wednesday thursday friday saturday sunday; do
    echo "  $(date -d "this $day" '+%Y-%m-%d %A')"
done

echo ""
echo "=== First and Last of Month ==="
year=$(date '+%Y')
month=$(date '+%m')

# First day
first=$(date -d "$year-$month-01" '+%Y-%m-%d (%A)')
echo "First of month: $first"

# Last day
last=$(date -d "$year-$month-01 +1 month -1 day" '+%Y-%m-%d (%A)')
echo "Last of month: $last"

# Days in month
days_in_month=$(date -d "$year-$month-01 +1 month -1 day" '+%d')
echo "Days in month: $days_in_month"

echo ""
echo "=== First Monday of Month ==="
first_of_month="$year-$month-01"
dow=$(date -d "$first_of_month" '+%u')  # 1=Monday

if ((dow == 1)); then
    first_monday="$first_of_month"
else
    days_until=$((8 - dow))
    first_monday=$(date -d "$first_of_month +$days_until days" '+%Y-%m-%d')
fi
echo "First Monday: $first_monday"

echo ""
echo "=== Check Weekend vs Weekday ==="
check_day() {
    local d=$1
    local dow=$(date -d "$d" '+%u')
    local name=$(date -d "$d" '+%A')
    
    if ((dow >= 6)); then
        echo "$d ($name) is a WEEKEND"
    else
        echo "$d ($name) is a WEEKDAY"
    fi
}

check_day "2025-11-18"
check_day "2025-11-22"
check_day "2025-11-23"

echo ""
echo "=== Practical: Next Occurrence ==="
# Find next occurrence of a specific day (e.g., 15th)
target_day=15
current_day=$(date '+%d')
current_month=$(date '+%Y-%m')

if ((10#$current_day < target_day)); then
    next_occurrence="$current_month-$(printf '%02d' $target_day)"
else
    next_occurrence=$(date -d "$current_month-01 +1 month" '+%Y-%m')-$(printf '%02d' $target_day)
fi

echo "Next ${target_day}th: $next_occurrence ($(date -d "$next_occurrence" '+%A'))"

echo ""
echo "=== Calendar for Current Month ==="
cal

Using natural language for intuitive date manipulation.

#!/bin/bash
# Relative and natural language dates

echo "=== Basic Relative Terms ==="
echo "now:        $(date -d 'now' '+%Y-%m-%d %H:%M:%S')"
echo "today:      $(date -d 'today' '+%Y-%m-%d')"
echo "yesterday:  $(date -d 'yesterday' '+%Y-%m-%d')"
echo "tomorrow:   $(date -d 'tomorrow' '+%Y-%m-%d')"

echo ""
echo "=== Time-of-Day Terms ==="
echo "midnight:   $(date -d 'midnight' '+%H:%M:%S')"
echo "noon:       $(date -d 'noon' '+%H:%M:%S')"
echo "teatime:    $(date -d 'teatime' '+%H:%M:%S')"  # 4:00 PM

echo ""
echo "=== Ago and Hence ==="
echo "3 days ago:      $(date -d '3 days ago' '+%Y-%m-%d')"
echo "2 weeks ago:     $(date -d '2 weeks ago' '+%Y-%m-%d')"
echo "1 month ago:     $(date -d '1 month ago' '+%Y-%m-%d')"
echo "1 week hence:    $(date -d '1 week hence' '+%Y-%m-%d')"
echo "fortnight:       $(date -d 'fortnight' '+%Y-%m-%d')"  # 2 weeks

echo ""
echo "=== Compound Expressions ==="
echo "yesterday noon:      $(date -d 'yesterday noon' '+%Y-%m-%d %H:%M')"
echo "tomorrow midnight:   $(date -d 'tomorrow midnight' '+%Y-%m-%d %H:%M')"
echo "next week monday:    $(date -d 'next week monday' '+%Y-%m-%d')"
echo "2 days ago 3pm:      $(date -d '2 days ago 15:00' '+%Y-%m-%d %H:%M')"

echo ""
echo "=== First/Last of Period ==="
# First of current month
echo "First of month:   $(date -d "$(date '+%Y-%m-01')" '+%Y-%m-%d')"

# Last of current month
echo "Last of month:    $(date -d "$(date '+%Y-%m-01') +1 month -1 day" '+%Y-%m-%d')"

# First of year
echo "First of year:    $(date '+%Y-01-01')"

# Last of year
echo "Last of year:     $(date '+%Y-12-31')"

echo ""
echo "=== Practical: Report Periods ==="
today=$(date '+%Y-%m-%d')

echo "Report periods as of $today:"
echo ""
echo "Today:        $today"
echo "Yesterday:    $(date -d 'yesterday' '+%Y-%m-%d')"
echo ""
echo "This week:    $(date -d 'last monday' '+%Y-%m-%d') to $(date -d 'next sunday' '+%Y-%m-%d')"
echo "Last week:    $(date -d 'last monday -7 days' '+%Y-%m-%d') to $(date -d 'last sunday' '+%Y-%m-%d')"
echo ""
echo "This month:   $(date '+%Y-%m-01') to $(date -d "$(date '+%Y-%m-01') +1 month -1 day" '+%Y-%m-%d')"
echo "Last month:   $(date -d 'last month' '+%Y-%m-01') to $(date -d "$(date '+%Y-%m-01') -1 day" '+%Y-%m-%d')"
echo ""
echo "Last 7 days:  $(date -d '-6 days' '+%Y-%m-%d') to $today"
echo "Last 30 days: $(date -d '-29 days' '+%Y-%m-%d') to $today"
echo "Last 90 days: $(date -d '-89 days' '+%Y-%m-%d') to $today"

echo ""
echo "=== Quarter Calculation ==="
month=$(date '+%-m')
quarter=$(( (month - 1) / 3 + 1 ))
year=$(date '+%Y')

q_start_month=$(( (quarter - 1) * 3 + 1 ))
q_end_month=$((quarter * 3))

q_start=$(date -d "$year-$q_start_month-01" '+%Y-%m-%d')
q_end=$(date -d "$year-$q_end_month-01 +1 month -1 day" '+%Y-%m-%d')

echo "Current quarter: Q$quarter $year"
echo "Q$quarter: $q_start to $q_end"

Creating dated filenames and implementing rotation schemes.

#!/bin/bash
# Log rotation and file dating

echo "=== Filename Timestamp Patterns ==="
echo "Second:    file-$(date '+%Y%m%d_%H%M%S').log"
echo "Minute:    file-$(date '+%Y%m%d_%H%M').log"
echo "Hourly:    file-$(date '+%Y-%m-%d_%H').log"
echo "Daily:     file-$(date '+%Y-%m-%d').log"
echo "Weekly:    file-$(date '+%Y-W%W').log"
echo "Monthly:   file-$(date '+%Y-%m').log"
echo "Yearly:    file-$(date '+%Y').log"

echo ""
echo "=== Rotation File Names ==="
base="application"

echo "Current:   ${base}.log"
for i in {1..7}; do
    rotated="${base}.log.$(date -d "-$i days" '+%Y-%m-%d')"
    echo "Day -$i:    $rotated"
done

echo ""
echo "=== Backup Naming Schemes ==="
prefix="backup"

echo "Daily:     ${prefix}-daily-$(date '+%Y-%m-%d').tar.gz"
echo "Weekly:    ${prefix}-weekly-$(date -d 'last sunday' '+%Y-%m-%d').tar.gz"
echo "Monthly:   ${prefix}-monthly-$(date '+%Y-%m').tar.gz"
echo "Quarterly: ${prefix}-Q$(( ($(date '+%-m') - 1) / 3 + 1 ))-$(date '+%Y').tar.gz"
echo "Yearly:    ${prefix}-$(date '+%Y').tar.gz"

echo ""
echo "=== Rotation Script Pattern ==="
cat << 'SCRIPT'
#!/bin/bash
LOG_DIR="/var/log/myapp"
LOG_FILE="$LOG_DIR/app.log"
KEEP_DAYS=7

# Rotate current log
if [[ -f "$LOG_FILE" ]]; then
    mv "$LOG_FILE" "${LOG_FILE}.$(date '+%Y-%m-%d')"
    touch "$LOG_FILE"
    chmod 644 "$LOG_FILE"
fi

# Remove old rotated logs
find "$LOG_DIR" -name "app.log.*" -mtime +$KEEP_DAYS -delete

echo "Rotation complete: $(date)"
SCRIPT

echo ""
echo "=== Find Files by Date ==="
echo "Modified in last 24 hours:"
echo "  find /var/log -mtime -1 -type f"
echo ""
echo "Modified more than 7 days ago:"
echo "  find /var/log -mtime +7 -type f"
echo ""
echo "Modified today:"
echo "  find /var/log -daystart -mtime 0 -type f"
echo ""
echo "Modified between dates:"
echo "  find /var/log -newermt '2025-11-01' ! -newermt '2025-11-15'"

echo ""
echo "=== Cleanup Schedule ==="
echo "Retention policy cleanup dates:"
printf "%-20s %s\n" "Hourly logs:" "$(date -d '-24 hours' '+%Y-%m-%d %H:%M')"
printf "%-20s %s\n" "Daily backups:" "$(date -d '-7 days' '+%Y-%m-%d')"
printf "%-20s %s\n" "Weekly backups:" "$(date -d '-4 weeks' '+%Y-%m-%d')"
printf "%-20s %s\n" "Monthly backups:" "$(date -d '-12 months' '+%Y-%m-%d')"
printf "%-20s %s\n" "Yearly backups:" "$(date -d '-7 years' '+%Y-%m-%d')"

echo ""
echo "=== Date-Based Directory Structure ==="
# Create hierarchy: /logs/2025/11/18/
log_dir="/tmp/logs/$(date '+%Y/%m/%d')"
echo "Creating: $log_dir"
mkdir -p "$log_dir"
ls -la /tmp/logs/

Working with cron schedules and finding next run times.

#!/bin/bash
# Cron schedule helpers

echo "=== Current Time Context ==="
echo "Now:       $(date '+%Y-%m-%d %H:%M:%S %A')"
echo "Hour:      $(date '+%H')"
echo "Minute:    $(date '+%M')"
echo "Day:       $(date '+%d')"
echo "Month:     $(date '+%m')"
echo "Weekday:   $(date '+%u') ($(date '+%A'))"

echo ""
echo "=== Next Run Calculations ==="

# Daily at 2 AM
current_hour=$(date '+%H')
if ((10#$current_hour < 2)); then
    next_daily=$(date -d 'today 02:00' '+%Y-%m-%d %H:%M')
else
    next_daily=$(date -d 'tomorrow 02:00' '+%Y-%m-%d %H:%M')
fi
echo "Daily @2AM:     $next_daily"

# Weekly on Sunday at 3 AM
current_dow=$(date '+%u')
if ((current_dow == 7 && 10#$current_hour < 3)); then
    next_weekly=$(date -d 'today 03:00' '+%Y-%m-%d %H:%M')
else
    next_weekly=$(date -d 'next sunday 03:00' '+%Y-%m-%d %H:%M')
fi
echo "Weekly Sun@3AM: $next_weekly"

# Monthly on 1st at midnight
current_day=$(date '+%d')
if ((10#$current_day == 1 && 10#$current_hour < 1)); then
    next_monthly=$(date -d 'today 00:00' '+%Y-%m-%d %H:%M')
else
    next_monthly=$(date -d "$(date -d '+1 month' '+%Y-%m-01') 00:00" '+%Y-%m-%d %H:%M')
fi
echo "Monthly 1st:    $next_monthly"

# Every 15 minutes
current_min=$(date '+%M')
next_quarter=$((((10#$current_min / 15) + 1) * 15))
if ((next_quarter >= 60)); then
    next_15min=$(date -d '+1 hour' '+%Y-%m-%d %H:00')
else
    next_15min=$(date '+%Y-%m-%d %H:')$(printf '%02d' $next_quarter)
fi
echo "Every 15min:    $next_15min"

echo ""
echo "=== Cron Expression Examples ==="
cat << 'EOF'
# ┌───────────── minute (0 - 59)
# │ ┌───────────── hour (0 - 23)
# │ │ ┌───────────── day of month (1 - 31)
# │ │ │ ┌───────────── month (1 - 12)
# │ │ │ │ ┌───────────── day of week (0 - 6, Sunday=0)
# │ │ │ │ │
# * * * * *

0 2 * * *     # Daily at 2:00 AM
0 3 * * 0     # Weekly Sunday at 3:00 AM
0 0 1 * *     # Monthly on 1st at midnight
*/15 * * * *  # Every 15 minutes
0 9-17 * * 1-5 # Hourly 9-5, Mon-Fri
0 0 * * 1     # Weekly Monday at midnight
EOF

echo ""
echo "=== Time Until Next Run ==="
next_run=$(date -d 'tomorrow 02:00' '+%s')
now=$(date '+%s')
until_secs=$((next_run - now))
until_hours=$((until_secs / 3600))
until_mins=$(( (until_secs % 3600) / 60 ))

echo "Next daily backup (2 AM):"
echo "  Runs at: $(date -d @$next_run '+%Y-%m-%d %H:%M')"
echo "  In: ${until_hours}h ${until_mins}m"

echo ""
echo "=== Check if Should Run Now ==="
# Simulate cron check for "0 * * * *" (hourly on the hour)
should_run_hourly() {
    local minute=$(date '+%M')
    ((10#$minute == 0))
}

# Simulate check for "0 2 * * *" (daily at 2 AM)
should_run_daily_2am() {
    local hour=$(date '+%H')
    local minute=$(date '+%M')
    ((10#$hour == 2 && 10#$minute == 0))
}

# Simulate check for weekday only
is_weekday() {
    local dow=$(date '+%u')
    ((dow <= 5))
}

echo "Current minute: $(date '+%M')"
should_run_hourly && echo "Hourly job: SHOULD RUN" || echo "Hourly job: not yet"

echo "Current hour: $(date '+%H')"
should_run_daily_2am && echo "Daily 2AM job: SHOULD RUN" || echo "Daily 2AM job: not yet"

is_weekday && echo "Weekday check: YES" || echo "Weekday check: NO (weekend)"

A reusable library of date/time functions for scripts.

#!/bin/bash
# Complete date/time function library

#######################################
# Date Parsing and Formatting
#######################################

# Parse any date to epoch
to_epoch() {
    date -d "$1" '+%s' 2>/dev/null
}

# Epoch to formatted date
from_epoch() {
    local epoch=$1
    local format=${2:-'%Y-%m-%d %H:%M:%S'}
    date -d @$epoch "+$format"
}

# Format date
format_date() {
    local input=$1
    local format=$2
    date -d "$input" "+$format"
}

#######################################
# Date Arithmetic
#######################################

# Add days to date
add_days() {
    local d=$1 n=$2
    date -d "$d +$n days" '+%Y-%m-%d'
}

# Subtract days
sub_days() {
    local d=$1 n=$2
    date -d "$d -$n days" '+%Y-%m-%d'
}

# Days between dates
days_between() {
    local d1=$1 d2=$2
    local e1=$(to_epoch "$d1")
    local e2=$(to_epoch "$d2")
    echo $(( (e2 - e1) / 86400 ))
}

# Add months
add_months() {
    local d=$1 n=$2
    date -d "$d +$n months" '+%Y-%m-%d'
}

#######################################
# Date Checks
#######################################

# Is valid date
is_valid_date() {
    date -d "$1" &>/dev/null
}

# Is weekend
is_weekend() {
    local dow=$(date -d "$1" '+%u')
    ((dow >= 6))
}

# Is in range
date_in_range() {
    local d=$1 start=$2 end=$3
    local de=$(to_epoch "$d")
    local se=$(to_epoch "$start")
    local ee=$(to_epoch "$end")
    ((de >= se && de <= ee))
}

#######################################
# Calendar Functions
#######################################

# First of month
first_of_month() {
    local d=${1:-$(date '+%Y-%m-%d')}
    date -d "$(date -d "$d" '+%Y-%m-01')" '+%Y-%m-%d'
}

# Last of month
last_of_month() {
    local d=${1:-$(date '+%Y-%m-%d')}
    date -d "$(date -d "$d" '+%Y-%m-01') +1 month -1 day" '+%Y-%m-%d'
}

# Days in month
days_in_month() {
    local d=${1:-$(date '+%Y-%m-%d')}
    date -d "$(date -d "$d" '+%Y-%m-01') +1 month -1 day" '+%d'
}

# Quarter of date
quarter_of() {
    local d=${1:-$(date '+%Y-%m-%d')}
    local month=$(date -d "$d" '+%-m')
    echo $(( (month - 1) / 3 + 1 ))
}

#######################################
# Duration Formatting
#######################################

# Format seconds as readable
format_duration() {
    local secs=$1
    local d=$((secs / 86400))
    local h=$(( (secs % 86400) / 3600 ))
    local m=$(( (secs % 3600) / 60 ))
    local s=$((secs % 60))
    
    if ((d > 0)); then
        printf "%dd %dh %dm" $d $h $m
    elif ((h > 0)); then
        printf "%dh %dm %ds" $h $m $s
    elif ((m > 0)); then
        printf "%dm %ds" $m $s
    else
        printf "%ds" $s
    fi
}

#######################################
# Demo
#######################################

echo "========================================"
echo "    Date/Time Library Demo"
echo "========================================"

echo ""
echo "=== Parsing and Formatting ==="
echo "to_epoch 'Nov 18, 2025': $(to_epoch 'Nov 18, 2025')"
echo "from_epoch 1731958245: $(from_epoch 1731958245)"
echo "format_date 'tomorrow' '%A %B %d': $(format_date 'tomorrow' '%A %B %d')"

echo ""
echo "=== Date Arithmetic ==="
echo "add_days '2025-11-18' 10: $(add_days '2025-11-18' 10)"
echo "sub_days '2025-11-18' 30: $(sub_days '2025-11-18' 30)"
echo "days_between '2025-01-01' '2025-11-18': $(days_between '2025-01-01' '2025-11-18')"
echo "add_months '2025-11-18' 3: $(add_months '2025-11-18' 3)"

echo ""
echo "=== Date Checks ==="
is_valid_date "2025-11-18" && echo "2025-11-18: valid" || echo "2025-11-18: invalid"
is_valid_date "2025-02-30" && echo "2025-02-30: valid" || echo "2025-02-30: invalid"
is_weekend "2025-11-22" && echo "2025-11-22: weekend" || echo "2025-11-22: weekday"

echo ""
echo "=== Calendar Functions ==="
echo "first_of_month: $(first_of_month)"
echo "last_of_month: $(last_of_month)"
echo "days_in_month: $(days_in_month)"
echo "quarter_of: Q$(quarter_of)"

echo ""
echo "=== Duration Formatting ==="
echo "3661 seconds: $(format_duration 3661)"
echo "90061 seconds: $(format_duration 90061)"

echo ""
echo "========================================"