Ok I hope you understand this question. I have a "due date" for something as a field, and I have a "completed date" field. I want if the due date is < completed date , print LATE.

Is that possible?

Yes. Make a calculation field with the following expression:

If [ due date < completed date, "LATE", "" ]

Make the calculation result text.

I've tried that but it won't work. I tried it in a script, and I tried doing auto enter calculation. It doesn't seem to be able to use a date as a calculation.

It cannot be a script, nor an auto-enter calculation. It needs to be a calculated field. It it will work. Keep trying.

Perhaps your date fields aren't really dates? That is, the field is defined as text, not date. If so, you have to change their data type, then re-enter all the data.

Thank you so much Vaughan, it finally worked, your a savior!