In your subVI, you've stripped all time information from the signal when you convert to a dbl. Keep the waveform data type if you want to record the timestamps. Instead of writing when the main VI finishes, you could simply put an Export Waveforms to Spreadsheet File or Write to Measurement File inside the main loop. If you want to save every 15 minutes, use an elapsed time function and wire the 'time has elapsed' output to a case statement. Put your file write inside the case statement. Use a shift register and a build array and pass this to your write function. After the write is complete, clear the array.
p.s. Your subVI is pretty inefficient because you are starting and stopping the task with each iteration. Start the task before the main while loop begins (outside the while loop) and stop the task when the main loop finishes (again, outside the loop).
