Has two modes:
if add= it specifies the number of seconds to be added to the iso time(s) that arrives in standard input. It then prints out the new iso date.
If other= is specified it subtracts off this time from those arriving in standard input and prints out the difference.
The default unit is seconds of time, but this can be changed to days, minutes, or hours by adding those keywords to the command line.
keys - will cause the the time difference to be prefaced by
sec=,min=,hour=,days depending on the unit selected.
Ephem index