You can always take it earlier, so if you’re moving to an earlier time, just take it at that earlier time.
If you’re moving it to a later time you need to gradually move your time while staying in your usage window.
For example if the time you take it is 7am and you want to start taking it at 6 pm, you’d need to gradually move it to 6 pm in order to stay protected.
•take it at 8 am
•then at 9 am
•and so on moving your time 1 - 2 hours later until you are at your new desired time.