From How to use dates and times in Excel:
Excel stores all dates as integers and all times as decimal fractions. With this system, Excel can add, subtract, or compare dates and times just like any other numbers, and all dates are manipulated by using this system.
In this system, the serial number 1 represents 1/1/1900 12:00:00 a.m. Times are stored as decimal numbers between .0 and .99999, where .0 is 00:00:00 and .99999 is 23:59:59. The date integers and time decimal fractions can be combined to create numbers that have a decimal and an integer portion. For example, the number 32331.06 represents the date and time 7/7/1988 1:26:24 a.m.
Some dates are being interpreted as their serial date numbers instead of their Gregorian dates. Instead of recalculating fields, I wonder if the issue would be solved by highlighting the entire column in Excel and explicitly setting the format to date.