The DATEDIF function can handle a variety of "date difference" calculations to calculate the difference between two dates in years, months, and days. DATEDIF takes 3 arguments: start date, end_date, and "unit", which controls which result is returned.
In this case, we want days ignoring years so we supply "yd" for unit. (For the full list of options, see the DATEDIF page).
Once configured, the function is fully automatic and returns a result in the unit requested.