My son’s friend in school was born on February 29. We could go through the time pedantry and claim that she is 1 year old (she is 6), or pick an arbritrary moment in time (say March 1, except for leap year) and get on with the party.
If you’re plotting a satellite course, the details matter, but many, if not most use cases require consistency over precision.
Indeed, your son's friend's age is an ambiguous notion, Hence the need of a more precise contract than a simple date.
It's unfortunate how few libraries exist to handle abstract datetime constructs, and how frequently programming environments screw this up. The only one I know of that doesn't mince concepts is Java 8 Time [1], where points-on-timeline timestamps are clearly separate from human-centric calendrical concepts like Time-of-Day (LocalTime) and Month-Day without a year, so there's an appropriate datatype to model lots of common use-cases of date math.
[1] https://docs.oracle.com/javase/8/docs/api/java/time/package-...
The tricky thing to realize is that this is not a datetime. This is more of a contract/condition saying "when localtime will be at this value" or better yet "when localtime will have exceeded this value for the first time" because you know someone will put a date on a leap second or missing DST hour at one point.