The best way to pass datetimes to SQLServer via ODBC is using the ODBC syntax:

$sth->bind_param($param_n, q/{ts '1998-05-11 00:00:00'}/);
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In your case you have an additional problem since the datetime parameters occur in a function so some MS SQL Server versions will be unable to describe the parameter correctly. As a result, if the parameter occurs in a function you might have to do the following to be sure it works:

$sth->bind_param($param_n, q/{ts '1998-05-11 00:00:00'}/, SQL_VARCHAR)
+;
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See ODBC Datetime Format.

I could go in to why using SQL_DATETIME is not a good idea but it would be rather lengthy. Perhaps I should write a FAQ on this for DBD::ODBC.