I see here that ISDATE is a system function:
However, when I do:
and @variable is a varchar variable, it says "Name or alias cannot be reserved word: ISDATE.
Here is the function I was trying to write:
I assume I must be using ISDATE wrong somehow? What is the proper way to use it?
A quick check of the code finds that it is registered as a reserved word, but there is no implementation for the ISDATE function, currently. I'm not sure how it got included in the list on that web page. The rest of the system functions listed are all supported (and the rest of the list on that page, as far as I noticed in scanning it)--except that FTS optimization of CONTAINS is currently broken in 5.x.
ISDATE is among some other date/time-related functions that we are considering adding support for in 5.2 (planned for late 2015).
If you really need it before then, you might be able to create an implementation in .NET code as a CLR function to use as a workaround.