Is there a function like isdate() for datetime2?

In SQL Server 2012, you can use TRY_CONVERT:

SELECT TRY_CONVERT(DATETIME2, '2012-02-02 13:42:55.2323623'),
       TRY_CONVERT(DATETIME2, '2012-02-31 13:42:55.2323623');

Results:

2012-02-02 13:42:55.2323623    NULL

Or TRY_PARSE:

SELECT TRY_PARSE('2012-02-02 13:42:55.2323623' AS DATETIME2),
       TRY_PARSE('2012-02-31 13:42:55.2323623' AS DATETIME2);

(Same results.)

Sorry that I don't have a clever answer for you for < SQL Server 2012. You could, I guess, say

SELECT ISDATE(LEFT('2012-02-02 13:42:55.2323623', 23));

But that feels dirty.

TRY_CONVERT documentation on Microsoft Docs
TRY_PARSE documentation on Microsoft Docs

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