It compares the date and time of your computer to one entered into the program. Then it calculates the amount of time between the two dates and times. Values are updated once every second. Daylight savings time is taken into consideration. Below the top line, each value is total, that is, Years is the total number of years until or since the event.
Months is the total number of months, et cetera.
If you want, you can enter dates directly into the box at the bottom of the form. However a much better way to enter a date is via the setup screen. Click on Setup on the main screen above and the setup dialog box opens. Use Daylight Savings Offset, if checked, will properly calculate the actual elapsed time between dates. You can select a previously entered date for use by pressing the down arrow in the Select a Date box.