There are many situations where it makes sense to enter a time stamp or today's date into your worksheet. Of course there are functions such as TODAY() and NOW() but these will recalculate automatically. If you want just to input a date or time, there's a couple of handy keyboard shortcuts you can use:
Press Ctrl-: (control and colon) to insert today's date.
Press Ctrl-; (control and semi-colon) to insert the current time.
NOTE: I've tested the above with Excel 2013 but it should work with other versions too.