Copy these properties from jmeter.properties file and add to user.properties file
#---------------------------------------------------------------------------
# Results file configuration
#---------------------------------------------------------------------------
# Timestamp format - this only affects CSV output files
# legitimate values: none, ms, or a format suitable for SimpleDateFormat
jmeter.save.saveservice.timestamp_format=ms
jmeter.save.saveservice.timestamp_format=yyyy/MM/dd HH:mm:ss.SSS
just comment or uncomment the line jmeter.save.saveservice.timestamp_format=yyyy/MM/dd HH:mm:ss.SSS for getting epoch time or human readable timestamp format.
Need to restart the jmeter after updating the user.properties file