How to Compare Dates in Bash

Quick Answer: Compare Dates in Bash

For ISO 8601 dates (YYYY-MM-DD), simple string comparison works: if [ "$date1" \< "$date2" ]. For any format, convert to epoch timestamps and compare: if [ $(date -d "$d1" +%s) -lt $(date -d "$d2" +%s) ]. The epoch method is most reliable for any format.

Quick Comparison: Date Comparison Methods

Method	Format	Flexibility	Reliability
String compare	ISO 8601 only	Low	High
Epoch compare	Any	High	Very high
File dates	File mtime	Limited	Good

Bottom line: Use epoch conversion for reliable date comparisons with any format.

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Compare dates in Bash to determine which date is earlier, later, or equal. Learn string comparison, epoch time, and file modification dates.

Method 1: Date String Comparison

Simple string comparison works for ISO 8601 format dates (YYYY-MM-DD):

date1="2026-02-21"
date2="2026-03-15"

if [[ "$date1" < "$date2" ]]; then
  echo "date1 is earlier"
else
  echo "date1 is later or equal"
fi

Output:

date1 is earlier

Note: String comparison only works reliably with YYYY-MM-DD format because digits compare correctly as strings.

Convert to Epoch (Seconds Since 1970)

For more robust comparison, convert dates to epoch time (seconds):

#!/bin/bash

date1="2026-02-21"
date2="2026-03-15"

# Convert to epoch seconds
epoch1=$(date -d "$date1" +%s)
epoch2=$(date -d "$date2" +%s)

if [ "$epoch1" -lt "$epoch2" ]; then
  echo "date1 is earlier"
elif [ "$epoch1" -gt "$epoch2" ]; then
  echo "date1 is later"
else
  echo "dates are equal"
fi

Output:

date1 is earlier

Compare File Modification Dates

Use -nt (newer than) and -ot (older than) operators:

# Check if file1 is newer than file2
if [ file1.txt -nt file2.txt ]; then
  echo "file1 is newer"
fi

# Check if file1 is older than file2
if [ file1.txt -ot file2.txt ]; then
  echo "file1 is older"
fi

# Check if file1 exists and is newer than file2
if [ file1.txt -nt file2.txt ] && [ -f file1.txt ]; then
  echo "file1 is newer and exists"
fi

Calculate Days Between Dates

#!/bin/bash

date1="2026-02-21"
date2="2026-03-15"

# Convert to epoch
epoch1=$(date -d "$date1" +%s)
epoch2=$(date -d "$date2" +%s)

# Calculate difference in seconds
diff_seconds=$((epoch2 - epoch1))

# Convert to days
diff_days=$((diff_seconds / 86400))

echo "Days between dates: $diff_days"

Output:

Days between dates: 22

Compare Dates with Times

#!/bin/bash

date1="2026-02-21 10:30:00"
date2="2026-02-21 14:15:30"

epoch1=$(date -d "$date1" +%s)
epoch2=$(date -d "$date2" +%s)

diff_seconds=$((epoch2 - epoch1))

# Convert to hours and minutes
hours=$((diff_seconds / 3600))
minutes=$(((diff_seconds % 3600) / 60))

echo "Difference: $hours hours and $minutes minutes"

Output:

Difference: 3 hours and 44 minutes

Practical Example: Backup Rotation

#!/bin/bash

# File: cleanup_old_backups.sh

backup_dir="$1"
days_to_keep="$2"

if [ -z "$backup_dir" ] || [ -z "$days_to_keep" ]; then
  echo "Usage: $0 <backup_dir> <days>"
  exit 1
fi

if [ ! -d "$backup_dir" ]; then
  echo "ERROR: Directory not found: $backup_dir"
  exit 1
fi

current_time=$(date +%s)

for file in "$backup_dir"/*; do
  if [ -f "$file" ]; then
    file_time=$(stat -c %Y "$file")
    age_seconds=$((current_time - file_time))
    age_days=$((age_seconds / 86400))

    if [ "$age_days" -gt "$days_to_keep" ]; then
      echo "Removing old backup: $(basename "$file") ($age_days days old)"
      rm "$file"
    fi
  fi
done

echo "Cleanup completed"

Usage:

$ chmod +x cleanup_old_backups.sh
$ ./cleanup_old_backups.sh ~/backups 30
Removing old backup: backup_20250815.tar (195 days old)
Cleanup completed

Check if Date is in the Past

#!/bin/bash

target_date="2026-02-20"
current_date=$(date +%Y-%m-%d)

target_epoch=$(date -d "$target_date" +%s)
current_epoch=$(date -d "$current_date" +%s)

if [ "$target_epoch" -lt "$current_epoch" ]; then
  echo "$target_date is in the past"
elif [ "$target_epoch" -gt "$current_epoch" ]; then
  echo "$target_date is in the future"
else
  echo "$target_date is today"
fi

Output:

2026-02-20 is in the past

Compare with Deadline

#!/bin/bash

deadline="2026-03-31"
project="ProjectX"

deadline_epoch=$(date -d "$deadline" +%s)
current_epoch=$(date +%s)

if [ "$current_epoch" -gt "$deadline_epoch" ]; then
  echo "ERROR: $project deadline has passed!"
  exit 1
else
  days_left=$(( ($deadline_epoch - $current_epoch) / 86400 ))
  echo "$project deadline: $days_left days remaining"
fi

Output:

ProjectX deadline: 37 days remaining

Compare Timestamps from Files

#!/bin/bash

# Compare log files by timestamp in filename
log1="app_2026-02-15_10-30.log"
log2="app_2026-02-21_14-45.log"

# Extract dates from filenames
date1=$(echo "$log1" | cut -d_ -f2)
date2=$(echo "$log2" | cut -d_ -f2)

epoch1=$(date -d "$date1" +%s)
epoch2=$(date -d "$date2" +%s)

if [ "$epoch1" -lt "$epoch2" ]; then
  echo "$log2 is newer"
else
  echo "$log1 is newer"
fi

Output:

app_2026-02-21_14-45.log is newer

File Test Operators

Operator	Meaning
-nt	newer than
-ot	older than
-ef	same file

if [ file1 -nt file2 ] && [ -f file1 ]; then
  echo "file1 exists and is newer"
fi

Common Mistakes

1. Not using epoch time - string comparison unreliable for different formats
2. Forgetting date -d requires GNU date - BSD date syntax differs
3. Integer arithmetic overflow - rarely happens with epoch time
4. Not handling timezones - epoch is always UTC
5. Comparing invalid dates - validate format first

Tips and Best Practices

• Use YYYY-MM-DD format for consistency
• Convert to epoch for reliable comparisons
• Always check if date conversion succeeds
• Use -nt/-ot for file modifications
• Remember epoch time is in UTC

Summary

Date comparison in Bash requires careful attention to format. String comparison works for ISO dates, but epoch conversion is more robust for calculations. Use file operators for modification times, and always handle timezone considerations in production scripts.