A date string value is any of the following:
A reference to a cell containing a valid date string. For example, a reference to a cell containing =”Dec 19, 2014”.
A valid date string enclosed within quotation marks. For example, “11/10/2013”.
A reference to a cell formatted as text that contains a valid date string.
Date values typed directly into a cell or included within quotation marks can take several forms, depending on the date format specified on your device. For example, if you’ve selected United States as your region, date values of “2/15/2015”, “02/15/2015”, “2-15-2015”, “02-15-2015”, “Feb 15, 2015”, and “February 15, 2015” are all equivalent.