The long date option adds the day of the week and writes out the month. So 1/1/19 will display as Tuesday, January 1, 2019. The date will display in the format you choose, no matter how you enter it.

Click the Short date dropdown menu to change the format for short dates (such as in the taskbar). Click the Long date ...

You can easily see the problem simply by entering 12/31/1899 and 1/1/1900 into Excel using the Long Date format, as pictured below—notice that the earlier date is displayed as text while the later ...

When you enter a date that's formatted with slashes, Excel will recognize it as a date. For example, Excel will automatically choose the date format for a cell that contains 11/11/19. You can set ...