!!Determines if a string value consist of a date
!Functionality:
This function returns the date if the passed string is a valid date, or NULL if it is not (in the specified format)

!Parameters:

|STRING |  The test value you want to test
|FORMAT |  The format of the 

!Returns: 
Date

!Errors:  
* None
Returns NULL if the value is not a date, or is not in the specified format


!Notes

* This is normally used in programming logic to ensure that a variable contain a date value only, so as to prevent other errors from occurring