One way is to build a waveform data type with Build Waveform function, add t0 and dt info, and use the Export Waveforms to Spreadsheet function. The dt, (delta time) should be readily available from the sampling rate of your data acquisition. You could also create an array of time values, append this array to your Y array for a 2D array and use the 2D input of Write to Spreadsheet file. There is a function in LabVIEW called WDT Get Waveform Time Array that shows how to create an array of time values.