Solution
Once the data has been written to a file that is not a standard measurement format (.TDMS, .LVM) it loses the format that LabVIEW uses to process timestamp information. Since it is now saved in a different format LabVIEW cannot process the data as date and time information and will display the result of a string to numeric data conversion.
If you want to display this information on a graph, you would need to do some string manipulation to get the data in the format you desire.