You are comparing the boolean PHP: isset - Manual
] return value with an empty string.
if (isset($_POST['time_Tlp'] ) == ""
which is never
PHP: is_numeric - Manual
] checks for any valid numeric value which includes floating point numbers and negative values which are also invalid for your case. A better solution would be using PHP: intval - Manual
]. But note that you have to handle the special cases "0" and "00" then.
Note also that the value 59 is valid (so use > 59 or >= 60).