It can vary by state and/or local law. Insurance providers and individual pharmacies have additional policies. Then some pharmacists seem to have additional rules, and in my experience, will say it is the law or insurance company policy. In my town, legally, a 7 day supply of hydro can be filled on the 5th day, a 30 day supply on the 25th day.

I only know this because I was researching something else in a nearby law library and started looking up other issues that bugged me.

The three Mom&Pop places in town stick to this. If your insurance requires an extra day or two, they will give it to you and file when the insurance will pay. On the other hand the Super Wal-mart will not fill a 7 day supply till the 8th day, a 30 day supply till the 31st day.