Fix timezone portion of string output of time_to_isoDateTimeStr()

The output of this function was not ISO-8601 complaint (blush) and was not
parsable by isoDateTimeStr_parse() (returned -1).

e.g. "20231217T011437-480" rather than "20231217T135222-0800" (in PST)

Positive UTC offset (eastern) zones would be even worse (the + separator
would be missing).