Bash: subtracting 10 mins from a given time

Question

In a bash script, if I have a number that represents a time, in the form hhmmss (or hmmss), what is the best way of subtracting 10 minutes?

ie, 90000 -> 85000

Answer 1

why not just use epoch time and then take 600 off of it?

$ echo "`date +%s` - 600"| bc; date 
1284050588
Thu Sep  9 11:53:08 CDT 2010
$ date -d '1970-01-01 UTC 1284050588 seconds' +"%Y-%m-%d %T %z"
2010-09-09 11:43:08 -0500