Number of Days Remaining Before a Date
This formula calculates the number of days remaining before a date:
=your_date_here-TODAY()
Example of Use
In this example, the goal is to obtain the number of days before the date in cell B1.
Enter the formula:
=B1-TODAY()

You will likely get something like this:

Then simply change the cell format to get a number instead of a date:

The number of days remaining will be automatically updated thanks to the TODAY function.