For doing this calculation, you need first to create an additional field with type DATE, where to store the info.
Then, the formula to calculate the days between the date provided and a specific date is:
int([AdditionalField]-#1900-01-01#)
Remember that this formula calculates the difference in days between the two dates.
If you want to know the number of days, you need to add a +1 in the formula.
This calculation could be useful when you want to check if a Medical Certificate is less than a year old than the Event Date.