RAT stores the date and time of each sample in the 'DateTime' column using the following format:
"mmm" represents milliseconds. For example, like this: 7/10/20 13:57:29.034
However, if you open a RAT data file in Excel unexpected things can happen. My version of Excel defaults to showing just the minutes and seconds, which is not super helpful.
To show the full data and time, highlight the 'DateTime' column and right click to select "Format Cells". In the window that pops up, highlight "Custom" as the format and copy/paste the following format into the box called "Type":
[$-en-US]m/d/yy h:mm:ss.000 ;@
Excel should now show the full date and time, including milliseconds, for each frame.