My current project has modest needs; there are some times in a database but I just make sure everything is handled as UTC internally, and converted on rendering

My current project has modest needs; there are some times in a database but I just make sure everything is handled as UTC internally, and converted on rendering (showing the timezone explicitly). There are some timedeltas (Python stdlib) involved, but they are expressed in days. As you notice, “one month from now” is ambiguous, and I think it is good that timedelta does not use that unit. So I haven't needed to go outside Python's stdlib (which mirrors a POSIX libc), even though I don't hesitate to do so. In other projects I've used clock_monotonic (POSIX, but needs a Python module or ctypes until the next Python 3 release), and either dateutil (if I need to do more advanced calendar math) or pytz (if I just need a timezone database).