Our softvision colleagues have asked that the Nagbot tool be updated to use working days as well as calendar days. I own that thing, and will do so directly. Not sure if this is the right category for this, but I'm self-assigning it.
are you referring to the request reminder emails (ie request nagging), or is "nagbot" something else? it isn't a term used within the BMO codebase afaict. request reminders already ignore weekends when calculating durations (see https://bugzilla.mozilla.org/show_bug.cgi?id=984438#c6). "working days" is trickier; public holidays would need to be excluded, and those are varied depending on location (not timezone), and fluid.
Nagbot is a little SLA-notification robot I maintain that looks for untriaged bugs and brings them to the attention of an increasingly-important set of people as they age, it's not a Bugzilla-native thing. Sorry, for the confusion, I just wasn't sure where else to put this.