Instead of using the "Export Waveforms to Spreadsheet" VI which converts the time stamp into time/date format, you could write the time data to file as the raw time stamp, which is seconds since Jan1,1903. Excel should have no trouble calculating the relative time from that as long as you treat it as a number rather than a time format. Alternatively, you can generate the relative time in LabVIEW and write that to file.