How would I format a date in a table such that the next row cannot have a date that precedes the current row?

For example, I have a table of 2 columns which have column headings "Date" and "Employee" and my first row (not the heading row) has the date as 20/10/2016. How do I ensure the next row cannot have a date that precedes 20/10/2016? (i.e. the next row's date must be the same or a date after 20/10/2016).

Thus if I have three rows of date values as 20/10/2016 and I start a new date 21/10/2016 then every succeeding row must be greater than or equal to 21/10/2016