If the syntax isn’t obvious without looking at the manual, you might as well use an actual standardized syntax. For months (without context, and where you need an absolute interval), I just interpret it as 4 weeks.
If the syntax isn't obvious, you should use however much verbosity it takes to make it obvious, because people will still ignore the standard and make mistakes.
Funny, I would interpret a month as 30 days (since the lunar cycle is slightly over 29.5 days).