Checking for dates in Excel can be a very interesting problem as Excel stores dates as numbers. For example, 4-Jul-15 is stored as 42189.
To elaborate further on this, 1-Jan-1900 is treated as 1. The difference between a date and 1-Jan-1900 +1 is the serial number of that date. hence, in case of 4-Jul-15, it is 42189.
Hence, if you write 42189 and 4-Jul-15 in two different cells, Excel has no built-in mechanism to differentiate between these two.