Master Bash Time Arithmetic: Add, Subtract, and Compute Date Differences

Background

When writing shell scripts, date and time calculations such as adding intervals or computing the difference between two timestamps can be performed using Unix timestamps and arithmetic expansion.

Time addition

Convert the base date to a Unix timestamp

time1=$(date +%s -d '1990-01-01 01:01:01')
echo $time1   # 631126861

Convert the interval to seconds

time2=$((1*60*60 + 20*60))   # 1 hour 20 minutes = 4800 seconds
echo $time2   # 4800

Add the timestamps

time1=$((time1 + time2))
echo $time1   # 631131661

Convert back to a readable date

date -d "@${time1}" '+%Y-%m-%d %H:%M:%S'
# 1990-01-01 02:21:01

Time difference calculation

Convert each date to a Unix timestamp and subtract

timeDiff=$(( $(date +%s -d '2010-01-01') - $(date +%s -d '2009-01-01 11:11:11') ))
echo $timeDiff   # seconds between the two dates

Convert the result to larger units if needed

echo $((timeDiff / 60))      # minutes
echo $((timeDiff / 3600))    # hours

Shell arithmetic operators

Arithmetic expansion $(( )) evaluates expressions. Example:

a=$((1+2))
echo $a   # 3

expr command provides equivalent functionality in older shells. Example:

a=$(expr 1 + 2)
echo $a   # 3