I think you need to count syns as to whatever 'your' day is.
So, if you sleep in the day and work at night, that's your day!
I would say, a dieting day starts after a period of sleeping and when you have your first meal. However, if you sleep at odd times ie different times of the day or night, it might be easier to just count 24 hour period as a dieting day.
Hope that makes sense!!!