I’m often surprised to see how stores and other businesses post their hours of operation. Usually this is a simple matter, e.g. “We’re open 10-8 Mon through Sat, and 12-6 on Sun”. Aside from the question of which day starts the week (I’m partial to Monday rather than Sunday), posting these hours on your front door or on your website is a widely accepted and effective practice.

But 24 hour businesses, public services, and even web-based applications present their own challenges, since being closed is the exception, rather than the rule. For example, one of Wal-Mart’s 24 hour stores provides a simple sign that says “Open 24 hours a day, except on Christmas Eve and New Year’s Eve”. (Don’t quote me on those exact holidays, though.) This makes it easy for customers, who can count on the store always being open except for those few holidays.

In contrast, I saw a public transit system with a sign saying “Trains run to downtown from 4 am – 1 am”. Basically, they’re saying you can catch a train anytime, as long as it’s not between 1 am and 4 am. This is a lot to compute when you only spend a second or two reading the sign. Perhaps a better approach would be to say, “Train service to downtown is available throughout the day and night, except for 1 am to 4 am in the early morning”. This makes it much easier to compare your scheduled travel time to see if it falls into the brief window whem service isn’t available.

To generalize this a bit, if your service is available a lot more often than it’s not, and people generally expect it to be available, you should consider stating your hours like the Wal-Mart store mentioned above. Tell customers when you’re NOT open, rather than when you are. This is clearly a judgement call, but I think it’s worth considering the next time you’re updating your hours.