I answer a lot of questions regarding date arithmetic and see a lot of over-complicated answers. This may in part be due to the poor MS help documentation. This is a summary of the main answers, from which you should be able to derive most other solutions.
All examples are given as a field definition in the query grid, but other usage (criteria, update to) should be obvious!
What is a date?
Access stores date/time variables like floating point numbers where the integer part represents days since 30/12/1899 and the fractional part represents time of day.
You can use this fact to do arithmetic without any functions in many cases. You need functions if your aritmetic needs to know about the calendar or to represent the results in the way you want.
to get a date N days before/after a given date [givendate] just use + and -:
if you want to return a date as "Friday", "Tuesday" etc. use the Format() function:
To group records in months in a totals query add a field:
and group by that field.
If you sum entries in a date/time field that represent times you get a numeric result, e.g. if you have
2:34 3:45 2:50
the sum comes to 0.38125. To get a H:M:S result you might use Format([mytimes],"hh:nn:ss") ("nn" for minutes not "mm" which is month) but this gives the hours modulo 24. This adds a position for the number of days if your sum is going to exceed 24 hours: