If your worksheet has dates that were perhaps imported or pasted that end up looking like a series of numbers like in the picture below, you probably would want to reformat them so they appear as either short or long dates. The date format will also be more useful if you want to filter, sort, or use it in date calculations.
Select the cell, cell range, or column that you want to reformat.
Click Number Format and pick the date format you want.
The Short Date format looks like this:
The Long Date includes more information like in this picture:
You can also do this by using the DATEVALUE function.