Please, review https://wiki.openstreetmap.org/wiki/Key:opening_hours before adding the opening_hours=* tag. If you don’t follow the syntax on the wiki, the software that actually uses the data just sees gibberish.

True examples of bad data I’ve just cleaned up today, with my comments after:

• opening_hours=Varies (use “unknown” instead if truly unknown, or open-end syntax if variable beyond a fixed known range)
• opening_hours=MWF 08:00-17:00; TT 10:00-19:00 (this can’t be automatically fixed by JOSM’s validator and has to be fixed by hand)
• opening_hours=Closed since 2008 (nope, this isn’t valid per the syntax either. “closed” by itself, maybe, but in that case the amenity/shop/office should probably just be untagged with a note in description=*).
• opening_hours: 08:00-24:00 pharmacy Mo-Fr 10:00-18:00; Sa 12:00-18:00 (either tag the pharmacy itself on a separate node, or use something like opening_hours:pharmacy=*)

Note that iD will not warn you of opening_hours syntax violations (in fact, as best I remember it will not check anything), it will happily let you put any kind of garbage you want in there. Maybe it should, though? How should we raise awareness that one can’t just slop any old junk in opening_hours in order for it to remain useful?