My coworker is suggesting a change to “office hour” in this case. Is that more right?
I know “hour” as a unit should be singular when (and only when) there’s exactly one, but as part of the name for hours of operation – e.g. “office hours” or “visiting hours” or “member hours” or simply “hours” – the singular form seems strange to me.
I could see using singular if there’s an implicit assumption that the referenced period of time is now, and will always be, exactly one hour, like an inherent feature, to the point that you could drop the end of the range as it would be implied (e.g. “Office hour: 9am”). In our case, the period has been longer in the past, and may be again in future, so it feels more correct to use “hours” to implicitly accommodate that possibility.
As for the other way around – phrases that use the singular “hour” for periods that are nonspecific or not exactly one hour, such as “dinner hour” or “witching hour” – this comment suggests that correct usage here is simply idiomatic, so perhaps that’s the case for “office hour(s)” too? [Edit: My examples here may be bogus, since I realized e.g. “dinner hour” can be a non-durational point in time, like an appointment; and “witching hour” is technically exactly one hour, though I think common usage might’ve moved away from that.]