Yes, it is proper to use true hyphens.
From ISO-8601: In representations the following characters are used as separators: [-] (hyphen): to separate the time elements “year” and “month”, “year” and “week”, “year” and “day”, “month” and “day”, and “week” and “day”;
In an environment where use is made of a character repertoire based on ISO/IEC 646, “hyphen” and “minus” are both mapped onto “hyphen-minus”