From LimeSurvey Manual
This option is important when you want to record timestamps together with the survey participants' answers. If you click on it, a drop-down list will show up with a multitude of date formats. Choose the one that fits your survey respondents. Do not forget that different date formats are used across the world. For example, it is recommended to use a different time format if your questionnaire focuses on the US market (mm-dd-yyyy) and not on the British market (dd-mm-yy).