Hence the way I would do it (and have for other purposes), as stated in my final sentence. Have the human state the intent and convert to your own internally preferred units as needed.
No no no, see now we just say "computer! do tedious math!", and it will do some slightly different math for us and compliment us on having asked it to do so.
The one true unit of time is hexadecimal encoded nanoseconds since the unix epoch. (I'm only half joking because I actually have authored code that used that before.)