If the Jitte leaves the battlefield after the "+2/+2" mode is announced but before it resolves, the bonus is given to the creature that was most recently equipped once the ability resolves.

Umezawa's Jitte's activated ability generates a modal choice. The choice is made when the ability is activated.

If the "target creature gets -1/-1 until end of turn" mode is chosen, the target creature must also be announced.

The ability can be used any time Umezawa's Jitte's controller has priority — only the "target creature" choice has additional requirements. Choosing the "Equipped creature gets +2/+2 until end of turn" mode does nothing if the Jitte isn't equipped to a creature when the ability resolves.

If the Jitte is moved after the "+2/+2" mode is announced but before it resolves, the bonus is given to the creature that is equipped when the ability resolves.