Merge pull request #171 from LibraryCarpentry/kylamj-updatelink

Update 03-dates-as-data.md to update link to Microsoft's explanation of Excel dates

Author: kylamj

episodes/03-dates-as-data.md

@@ -109,7 +109,7 @@ from before and after this date, Excel will translate only the post-1900 dates i
 If you're working with historic data, be extremely careful with your dates!
 
 Excel also entertains a second date system, the 1904 date system, as the default in Excel for Macintosh. This system will assign a
-different serial number than the [1900 date system](https://support.microsoft.com/en-us/help/214330/differences-between-the-1900-and-the-1904-date-system-in-excel). Because of this,
+different serial number than the [1900 date system](https://support.microsoft.com/en-us/office/date-systems-in-excel-e7fe7167-48a9-4b96-bb53-5612a800b487). Because of this,
 [dates must be checked for accuracy when exporting data from Excel](https://uc3.cdlib.org/2014/04/09/abandon-all-hope-ye-who-enter-dates-in-excel/) (look for dates that are about 4 years off).