Updated 24hour detection

Updated 24hour detection to look for lower case 'h' (12-hour clock hours) as well as case-insensitive 'a' (AM/PM marker), based on suggestions in #482
That's what I get for blindly following the format of the existing code ;)

They should both either be `<0`, `==-1` or `===-1` (I don't think the strict checking is necessary in this case, but valid formats could have the 'a' or 'h' in the first (0) position.