The program will attempt to interpret dates in the GEDCOM file as one of the normal types specified by the standard. If successful, these dates are written to the web pages in "human-readable" form, for example, "1 January 1900". If the date is not in a standard format, it is written to the web page just as it appears in the GEDCOM file, though the program does try to recognise which year it refers to, so that it can work out the correct order for entries in the index pages.
The program can interpret dates in the four calendars specified in the GEDCOM standard: Gregorian (the default calendar), Julian (or "old style"), Hebrew and French Revolutionary (though whether the last one has ever been used in a real GEDCOM file I don't know!). If a date is expressed in terms of one of these non-default calendars, the web pages will indicate which calendar has been used.
Most users should not need to adjust the default settings for the handling of dates. But for those with special requirements, the program does allow the user to change the settings for the standard calendars, or to add a non-standard calendar. To change the calendar settings, click on the "Edit calendars" item of the "Tools" menu.
In the dialogue box, to add a new calendar, click on "Add calendar". Then type into the "Calendar name" box the code for the calendar that appears in the GEDCOM, and into the "Interpretation" box the description of the calendar that's you'd like to appear on the web pages. Then specify the months of the year in order. For each one, click "Add month", then type into the "Abbreviation" box the name of the month as it appears in the GEDCOM, and into the "Interpretation" box the name of the month as you'd like it to appear on the web pages. When finished, click "OK" to save the changes.
If you would also like to save these changes for future use, you can do so in two different ways. click on "Save tag information as default" in the "Tools" menu.