Look at the NETWORKDAYS() function: will that do it for you? Your Jan
1 through Jan 10 example includes two weekends, so NETWORKDAYS will not
be your best choice if you count weekend days as holidays.

If you say "I'll be on holiday from the 7th through the 10th" then
you'll be on holiday for 4 days. To express those 4 days
mathematically, the formula is 10 minus 7 plus 1. You can make your
original formula work by adding 1.