#variables
time=2:00pm
date=2018-1-16

#I need to subtract 4 hours from the time variable
at -f /home/JS/filename $time-4hours $date >attemp 2>&1

$ echo true | at 2:00pm 2018-01-18 - 4 hours
warning: commands will be executed using /bin/sh
job 10 at Thu Jan 18 10:00:00 2018

$ at -f ... $time $date - 4 hours