Documented or not, it _is_ the function to use in my case. It avoids any date arithmetic, and is therefore much more efficient.

Integer iMs
UBigInt biStart 
Move (GetTickCount64()) to biStart
... something that takes time ...
Move (GetTickCount64()-biStart) to iMs
Function GetTickCount64 (that returns a UBigInt) is defined in WinKern.pkg and therefore always available.

GetTickCount64 should be preferred over GetTickCount for the presumably rare event when the application runs for more than 49.7 days straight. That is when the latter will loop or something.