After time is called at the end of a match, what happens if I use Dialga GX's "Timeless GX" attack during the "plus three turns" part?
If you use Timeless GX when time is called during your turn, you would then get "Turn 1" instead of your opponent. Likewise, if you use it during Turn 1 you would also get Turn 2 and your opponent would get Turn 3, and so on... However, if you use it on the final turn of the match you do not get an extra turn.
Source: Ultra Prism FAQ (2018-01-19), TPCi Rules Team (2018-02-01)

Relevant cards

Note: All applicable cards may not be listed below.
SM5_100
SM5_146
SM5_164