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

My version of bash doesn't support -d or --date as used above. However, assuming a correctly 0-padded input, this does work

$ input_time=130503 # meaning "1:05:03 PM"

# next line calculates epoch seconds for today's date at stated time
$ epoch_seconds=$(date -jf '%H%M%S' $input_time '+%s')

# the 600 matches the OP's "subtract 10 minutes" spec. Note: Still relative to "today"
$ calculated_seconds=$(( epoch_seconds - 600 )) # bc would work here but $((...)) is builtin

# +%H%M%S formats the result same as input, but you can do what you like here
$ echo $(date -r $calculated_seconds '+%H%M%S')

# output is 125503: Note that the hour rolled back as expected.