Is it possible to choose when the time off will be activated in the calendar for the following year?

Of course, you can! When activating the time in which a specific time off can be requested, you can set the expiry date of the accrued time off. You can indicate a concrete day in the following year on which that time off will expire. 

Indeed! When setting an expiration date for the accumulated time, you must enter this year's date so employees can have access to next year's calendar.