Visual Basic for Applications Reference
Returns a Boolean value indicating whether an expression can be converted to a date.
The required expression argument is a Variant containing a date expression or string expression recognizable as a date or time.
IsDate returns True if the expression is a date or is recognizable as a valid date; otherwise, it returns False. In Microsoft Windows, the range of valid dates is January 1, 100 A.D. through December 31, 9999 A.D.; the ranges vary among operating systems.