You can get that information from standard solar tables available on the net. This link is to the tool configured for Belize. Simply enter the dates or date range and it will give you the precise time of sunrise and sunset for those dates. This is actually configured for Belize City, so there will be about 5 minutes of variance. If you really need to be precise, just adjust the latitude to 18 and longitude to -87 for San Pedro.
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