Data Validation: Comparing 2 timestamps - If time, then may not =, if string, then allowed

Hi There,

I'm having some trouble solving the following Data Validation Problem:

I have two columns H and I. Column H cells contain start-times [HH:MM:SS] and cells in column I contain end-times [HH:MM:SS].

However, cells in these two columns can also contain specific strings like "NA" / "Skipped". Only if the start- and end-time values are time values, the may not be equal to each other. However, if the cells in columns H or I contain any of the String values, they may equal each other. Otherwise it must show an error.

I have come this far:
=AND(OR(TEXT($I3,"hh:mm:ss")="NA",TEXT($I3,"hh:mm:ss")="Aborted",TEXT($I3,"hh:mm:ss")="Skipped",NOT(ISERROR(TIMEVALUE(TEXT($I3,"hh:mm:ss"))))),NOT(TEXT($H3,"hh:mm:ss")=TEXT($I3,"hh:mm:ss")))

The above formula works, but it does not allow the H+I cells to be the same in case they are string values... It stops any values compared in columns H+I from being the same, no matter what.

I will greatly appreciate any suggestions!

Thanks.