I'm going to assume you mean the precision in the saved file and that you're using Export Waveform To Spreadsheet File. If that's true, you can modify the subVI Waveform Time to Date Time String. In there, the Format Date/Time String has a format string of %H:%M:%S%u. The %u is the fractional seconds portion and you can change it to %<digits>u for additional precision. For example, %8u would give you 8 decimal places. Don'f forget to save the modified VIs as a different name and in a different location.
If you don't want t0 in the file, you can wire a False constant to the Header? (write header:T) input.
