I believe I read somewhere on the Royal site that a My Time reservation can be modified in the app the day of the reservation. Modifications include cancellation, # of people and time (dependent on availability).
We have 6:45pm reservations for each day of our upcoming Explorer sailing as 'placeholders', but we may end up 'tweaking' any one of those depending on shore excursion return times, friends joining us on a particular night, or outright cancel one if we need to.
I'm sure someone will correct me if I'm wrong on the information here..