Time variable comparison. IF T#3S

This isn't urgent but a TIME value is failing a comparison test for me.

t_delay    : TIME;		// Declaration
-----
IF t_delay <= 0 THEN
    t_delay := T#3S
END IF

If I modify the variable value to T#2S the IF sets it back to T#3S.

Internally the TIME variables are integer. Why would a compare not work as I intend?

Many thanks.