It appears you're trying to give DateValue
a string, but that's not what's happening. There may be more going on that I don't understand, so I'll just show you an Immediate window session which may contain something you can build on.
PT_BirthDate = #1923-6-1#
? PT_BirthDate
6/1/1923
? DateDiff("yyyy", PT_BirthDate, Date())
90
' this throws error #13: Type mismatch ...
? DateValue( (day([PT_BirthDate])/month([PT_BirthDate])/1900) )
' it will work when you give DateValue a string ...
? DateValue("1900-" & Month(PT_BirthDate) & "-" & Day(PT_BirthDate))
6/1/1900
' or consider DateSerial instead ...
? DateSerial(1900, Month(PT_BirthDate), Day(PT_BirthDate))
6/1/1900