If you are dealing with physics, like a satellite course, "wall time clock" is irrelevant. Wall time clock is a human construct that is ambiguous in many edge cases, dependent on position and local politics.
Indeed, your son's friend's age is an ambiguous notion, Hence the need of a more precise contract than a simple date.
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.