quickly count back from present time, usage intended to be within the scope of 24h
intended usecase: compiled executable in PATH for terminal access
program accepts time in formats: HH:mm and HH mm
usage examples:
> since 8 02 (current time: 1200)
< 04h 00m
> since 13:25 (current time: 1100)
< 21h 35m
for minutes, only zero-padded singledigit values are tolerated
failsafes will be added when deemed necessary
go build in project root, assuming go/bin is already in PATH. may vary with env and associated aliases