Getting "hard halts" on years > 2039 (RPG list)

So, I was handed a project the other day to fix a problem with a program
that gets a "hard halt" (their terminology) when the program encounters a
date with a year > 2039. Since it's been over 15 years since I've last had
to think about this type of problem, I *think* this has to do with the
1940-2039 window for 2-digit years. The fix is to change the edit (TEST
opcode, I'm guessing) so that it doesn't dump on a year > 2039.

I do not have an error message or a dump to work with. Not even actual
data. All I have is that the "program gave a hard halt when the next
payment date = 2040-06-17". (BTW, the lack of actual errors/dumps is
something I intend to address in our next programming team meeting. This
is, apparently, a common scenario for fix projects. I'm new here, so I
need to tread carefully.)

The immediate concern is to identify where this error is taking place. My
initial search is to look for TEST opcodes using the 'dtz-formats' *YMD,
*DMY, *MDY, & *JUL. The program has many different 6-digit date fields,
and even the term "next payment date" is ambiguous in this large 20+ year
old program.

Should the above search cover all of the bases? If not, can anyone suggest
what else to look for?

Are there still any free Y2K analysis tools available? In researching this
project, I was astounded by the number of 6-digit dates that pervade not
only the files used in this program, but throughout the system.

- Dan