9.4. Date and Time Calculations

Before you start writing date and time calculations, think about how FileMaker keeps track of dates and times (Section 6.1.3). FileMaker internally stores any date or time value as a single number that uniquely identifies every day and time of that day. Then, when it needs to display a date or time, it converts the number to a value people recognize, like "11/7/2009" or "10:23 AM". As with other numbers that it stores one way and displays another, FileMaker does the math on the stored value, and then converts it for your convenience.

This secret to date and time storage isn't just a technicality. It actually tells you a lot about how you can use dates and times in calculations. For example, without this knowledge, you could spend ages trying to write a calculation that gives you the first day of the month following the date an invoice is due. But it's actually pretty simple: