The patch looks good to me.  Just a few nitpicks: the local convention seems to be no underscores in helper functions such as gettmarg.  I would call is_valid_tm, "checktm" instead.  Also, predicate-like naming (is_...) suggests a function without side-effects, while is_valid_tm sets an error when it gets an invalid tm.  Also, please add missing comments above gettmarg and is_valid_tm.  Finally, please start the function body with a "{" on a new line.  Thank you for your submission.