Time window for 4-month sample / protein test

Only recently did I begin paying attention to the exact schedule on which the 4-month arrives.

I thought I had figured out, and that a center manager had verified, that the nominal date is 4 calendar months after your last successful sample, [potentially] MINUS 6 days. That would land the actual date on your first visit of the week when your nominal 4-month date falls.

I asked last week because I came in a bit low and wanted a reminder of how long I had to work on it before the sample date popped up. The screener told me it would be the nominal date minus 13 days. That's a big difference.

If you actually know (i.e. you're center staff and pay attention, or you're a serious donor who tracks this stuff) how this works, I would love to have my confusion cleared up.