Does Sundial of the Infinite end triggered abilities that are already on the stack?

Sundial of the Infinite reads: '{1}, {T}: End the turn. Activate only during your turn.' The 'end the turn' effect is defined by CR 719.1, which instructs players to follow the 'end the turn' procedure immediately upon resolution.

CR 719.1 states that when an effect ends the turn, the following happen in order: (1) all phases and steps of the current turn after the current step end, (2) all spells and abilities on the stack are exiled (they don't resolve), and (3) all other effects that would end at end of turn do so. Crucially, step 2 exiles everything currently on the stack — including triggered abilities that have already triggered and been placed on the stack.

This is the primary competitive use of Sundial: if a triggered ability with a negative delayed effect (like the 'sacrifice at end of turn' trigger on tokens created by Endless Ranks of the Dead variants, or notably the end-step sacrifice trigger from Borrowed Time-style effects) goes on the stack, you can activate Sundial in response to exile that trigger before it resolves.

Concrete example: You control a Zombie token created by a card that says 'sacrifice it at the beginning of the next end step.' That delayed trigger goes on the stack at the beginning of your end step. In response, you activate Sundial of the Infinite. The turn ends, the sacrifice trigger is exiled without resolving, and your token survives.

Note that Sundial must be activated while the triggered ability is on the stack (i.e., in response to it), since you must activate Sundial during your turn (CR 719.1, 502.3). Once you pass priority and the ability resolves, it's too late.