You can count the difference between today and a date by using the TODAY() function and subtraction.
If your date is earlier than today, you can use the following formula:
=TODAY() - 12/25/2017
If you want to count how many days are left until the future date, you can use the following formula.
=12/25/2019 - TODAY()
You can use addition and subtraction with dates because they are just numbers, which are formatted as dates. You can learn more about storing dates and time in Excel by reading the following article.