Sundial of the Infinite

The earliest that you can activate Sundial of the Infinite's ability is during your upkeep step, after abilities that trigger "at the beginning of your upkeep" have been put onto the stack but before they resolve.

If any triggered abilities do trigger during this process, they're put onto the stack during the cleanup step. If this happens, players will have a chance to cast spells and activate abilities, then there will be another cleanup step before the turn finally ends.

If Sundial of the Infinite's ability is activated before the end step, any "at the beginning of the end step"-triggered abilities won't get the chance to trigger that turn because the end step is skipped. Those abilities will trigger at the beginning of the end step of the next turn. The same is true of abilities that trigger at the beginning of other phases or steps (except upkeep).