You don't need the cast:
already returns a
value, so a cast is redundant.
The error message refers to a character string:
Conversion failed when converting date and/or time from Character string.
Which means that you are passing the DateTime value to your DB by concatenation strings:
string sql = "UPDATE MyTable SET Updated_At='" + _updated_at + "'";
And that is what is causing your problem because the string you pass is being interpreted incorrectly by SQL, And you should be glad it is, because the chances are it would have worked two days ago, but stored the wrong values into your DB.
Never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Always use Parameterized queries instead.
When you concatenate strings, you cause problems because SQL receives commands like:
SELECT * FROM MyTable WHERE StreetAddress = 'Baker's Wood'
The quote the user added terminates the string as far as SQL is concerned and you get problems. But it could be worse. If I come along and type this instead: "x';DROP TABLE MyTable;--" Then SQL receives a very different command:
SELECT * FROM MyTable WHERE StreetAddress = 'x';DROP TABLE MyTable;
Which SQL sees as three separate commands:
SELECT * FROM MyTable WHERE StreetAddress = 'x';
A perfectly valid SELECT
DROP TABLE MyTable;
A perfectly valid "delete the table" command
And everything else is a comment.
So it does: selects any matching rows, deletes the table from the DB, and ignores anything else.
So ALWAYS use parameterized queries! Or be prepared to restore your DB from backup frequently. You do take backups regularly, don't you?
Fix that throughout your app, and the problem you have noticed will go at the same time.