Capricious date constants in stored proc call

Question

It prints: The syntax of the string representation of a datetime value is incorrect without a reason, when I try to call a stored proc with DATE parameters from another stored proc.

Strange thing, presence or absence of unused variables affects the result.

this fails:

--<ScriptOptions statementTerminator="^"/>
CREATE PROCEDURE MYTESTDATE2 ()
    DYNAMIC RESULT SETS 1
P1: BEGIN ATOMIC
    DECLARE @FOO INTEGER;

    CALL MYTESTDATE1 (
'2014-07-30'                                    -- IN @DATE_OF_INSERT TIMESTAMP,             
);

END P1
^

This succeeds:

CREATE PROCEDURE MYTESTDATE2 ()
    DYNAMIC RESULT SETS 1
P1: BEGIN ATOMIC
    DECLARE @FOO INTEGER;

    SET @FOO = NULL;

    CALL MYTESTDATE1 (
'2014-07-30'                                    -- IN @DATE_OF_INSERT TIMESTAMP,             
);

END P1

This succeeds too:

CREATE PROCEDURE MYTESTDATE2 ()
    DYNAMIC RESULT SETS 1
P1: BEGIN ATOMIC
    CALL MYTESTDATE1 (
'2014-07-30'                                    -- IN @DATE_OF_INSERT TIMESTAMP,             
);

END P1