Data transformation

If you're trying to get the 4 digit year from an HL7-formatted time string (YYYYmmddHHMM), your method should look like this (using GetYear rather than DateTime as method name for clarity):

ClassMethod GetYear(pDate As %String) As %String
{
   If ($LENGTH(pDate) > 4)
   {
      Return $EXTRACT(pDate,1,4)
   }
   Return ""
}
}

You would then call that class as follows:

Set Year = ##class(CUSTSOM.Training.Functions).GetYear(source.GetValueAt("PIDgrpgrp(1).PIDgrp.PID:7.1"))

The variable Year should then contain the 4 digit year, or the empty string if the value in PID:7.1 is 4 characters or less.

I see two things here. First, in your Data Transformation, since PIDgrpgrp() repeats, you will need to specify a repetition in the parenthesis like PIDgrpgrp(1) for the first repetition.

<assign value='##class(CUSTOM.Training.Functions).DateTime(source.{PIDgrpgrp(1).PIDgrp.PID:7.1})' property='target.{PID:7.1}' action='set' />

Then, in your DateTime method, you will need to return the year. For example, if the DateTime was formatted YYYYMMDD:

ClassMethod DateTime(DateTime As %Integer)
 {
   Set Year = $EXTRACT(DateTime,1,4)
   Quit Year
 }