IMHO the only way to create a timestamp that registers when a particular FIELD has changed is to normalize this field into a separate table . . . in other words, a table that contains your Field and the PK. Then create a third field that auto-enters Modification timestamp or date . Otherwise any field you modify in your record of your table as is set up currently will trigger a Modification.
No, it is possible to create a field in the same table. Create a Timestamp field; with an auto-enter calculation; uncheck [ ] Do not replace contents. Use the Let function to just setup an internal variable to the field you want to monitor. So changing that field will update this timestamp field. It does not refer to a Modification field; it does it internally.
f = field you want to monitor;
I think Rick is right. I needed to have a timestamp update whenever a certain field was modified, specifically a comment field. Creating a new table with just the fields needed and then importing all the records into the new table. I set the relationship to be able to create records via the relationship. so far seems to work. I think this is what Filemaker recommends too.