What is the correct way to call DateTime.TryParse from F#?

Chris's answer is correct if you really need to pass a mutable DateTime by reference. However, it is much more idiomatic in F# to use the compiler's ability to treat trailing out parameters as tupled return values:

let couldParse, parsedDate = System.DateTime.TryParse("11/27/2012")

Here, the first value is the bool return value, while the second is the assigned out parameter.

Here's how to execute DateTime.TryParse in F#:

let mutable dt2 = System.DateTime.Now
let b2 = System.DateTime.TryParse("12-20-04 12:21:00", &dt2)

Where the & operator finds the memory address of dt2 in order to modify the reference.

Here's some additional information on F# parameter syntaxt.