You can use dates and time in your formulas just like any other value. For example, if cell A1 contained the entry 5/1/19 you could use the formula =A1+100 to calculate the date 100 days later, which is 8/9/19.
One very important thing to know about working with date and time functions: while Excel can display dates and times using just about any format, it stores dates as chronological numbers called serial values. So, when you think of dates as months, days, and years, such as May 1, 2019, Excel thinks of dates in terms of serial numbers, such as 36281.
Since the date and time formulas often return serial number values, you should format any cells with date or time formulas with date and time formats that you can easily understand.