If you happened to call uptimeMillis right when it wrapped, then yes it would affect your postAtTime call.
A signed long in Java has the range:
-9,223,372,036,854,775,807 to 9,223,372,036,854,775,807 (~9.2E18)
9.2E18 milliseconds is 292,277,266 years. If you are working on a space probe, you probably want to take this into consideration, otherwise you can probably get away with assuming it won't wrap in your lifetime.
The kicker for me is that the Android documentation for uptimeMillis claims
This clock is guaranteed to be
monotonic . . .
Then soon after they say that uptimeMillis will be reset due to variable wrapping - the exact opposite of a monotonic clock!