How do I check if a time field is within 5 seconds of another in XSL?

I have an interface between two pieces of software with Stylesheets between to transform and also validate certain data. A requirement has been altered so that rather than just checking two fields are equal before selecting, I now require a test where they are equal or no more the 5 seconds greater?

If anyone has a solution to this I would love to know about it.

Thanks

Simon