Yes, on the Pro plan and above. You can set the start and end in 30-minute steps.
They're applied at those exact times at checkout as well as in the cart. A gift that hasn't started yet or has already ended is blocked at checkout.

Good to know
- Across all active gift rules, you can use up to 5 different start and end times (a 0:00 start and a 23:59 end aren't counted).
- The gift stops at the end time (for example, 21:00 means it runs until 20:59).
Related
Set when a gift is offered