Hello!! I want to ask when to observe Uposatha day? Sometimes the full moon can be before 00:00 or sometimes after 00:00; I read that it’s dawn to dawn; does it mean, observation needs to be dawn to dawn between which the full moon occurs? or Does observation start from the day after the full moon started? Or is it flexible (as long as we observe rules that’s fine, either before or after)
p.s. I guess Buddha’s time doesn’t had our clock 
The day of the uposatha is not calculated by what we would call the astronomical moment of full moon. There is a system to calculate it that is slightly different for each monastic community. For example, here is a calendar for the Mahanikaya (Thailand)
https://forestsangha.org/community/calendars/year_planners
And here is Dhammayut:
In Sri Lanka there are the official government uposatha days (which I’m guessing is for Siam Nikaya). Other orders in SL will have slightly different days.
You might find this thread useful as well:
From Ajahn Thanissaro’s Buddhist Monastic code:
To enable the bhikkhus to determine the date of this observance, he relaxed the rule against their studying astrology (see Chapter 10), which in those days had not yet separated from astronomy, allowing them to learn as much astronomy as needed to calculate whether the full and new moons fell on the fourteenth or fifteenth of a particular fortnight. (“At that time people asked the bhikkhus as they were going for alms, ‘Which day of the fortnight is it, venerable sirs?’ The bhikkhus said, ‘We don’t know.’ The people criticized and complained and spread it about, ‘These Sakyan-son monks don’t even know enough to calculate the fortnight, so how will they know anything else that’s admirable?’”—Mv.II.18.1)
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