This is a fun one.

1. Use the undocumented msxsl:utc function, which checks / converts a date into a normalized representation.
   
   
   
   
2. So, what will happen if I test it with with a garbage date, say, the 29th of February of 2001, which isn't a leap year?
   
   
   
3. So just check if the result of the utc function returns empty string or not.
   

Expression

msxsl:utc(concat(my:year, "-02-29")) != ""