Try this link, Beulah:
Great question! I like to use this website to calculates dates and dates between days:
I use the duration-between-two-dates calculator shown above on this link when I sometimes add a person's age at death to their profile--but there are a number of good date calculators included on this page for adding or subtracting days, months, years and so forth.
You can book-mark this page to have it handy when needed, if you find it as useful as I do. :-)