German Rail Pass

The one-month travel period starts from the day that you have the pass activated at a train station, such as Sept. 20 - Oct. 19 (ending at midnight).

Reading this line from the DB website: https://www.bahn.com/i/view/overseas/en/prices/passes/german-rail-pass-flexi.shtml

• You choose the first and the last day of travel. If you purchase your German Rail Pass on the internet, you will be asked to fill in the validity dates directly online.

Sounds like you pick the time period, not just a calendar month. Try doing a dummy booking.

Of course, I and others might question if this is your best option. Tell us where you want to go (and how many traveling together) and we can see if there are any cheaper options. If you can live with committing to advance purchase (changeable for a fee) they are the best price and usually less than the cost of a pass-day. Also short trips using laender and verkehrsverbund day tickets are less then pass days.

Always the same ambiguity: a calender month or a period of 30 days...
The text » If you purchase your German Rail Pass on the internet, you will be asked to fill in the validity dates directly online« suggests the pass can start any day of a month and end the next month. An email to reiseportal@deutschebahn.com will be helpful.
But are you sure a rail pass is a good deal? They don't come cheap. If you know your travelling days, advance booking gives huge discounts on non-refundable tickets.

I concur with Sam and Tonfromleiden - don't buy the GRP until you're sure it's what you need - and to know that, you really should share the details of your itinerary (assuming you have one) with exact or rough travel dates for each journey, and number of travelers and their ages, so that we can help you evaluate the options. It's next to impossible on your own because the options are so numerous. Many people buy the GRP (which is marketed only to non-Europeans) when the options that Europeans use are actually better.

Also, if it turns out the GRP is what you need/want, you should know it is available in Germany from stations at airports and in larger cities - and there are good reasons for delaying purchase until you are there.

Thanks to everyone for the good information and advice.

Another question about the German rail pass flexi (I think this is what the OP was referring to). I was looking into the 10 days pass to be used over one month's time. The Bundesbahn site seems to imply that you have to decide when you purchase it, at a rail office in Germany in the situation I was looking at, when your dates of use are. Do they mean you have to pick all 10 travel days right then when you purchase it? This would cut down on the whole "flex" part of the ticket if you have to lock in your travel days right then and there.
Outside of that, the price of 520 euros for two people to have 10 travel days within a month seems like a pretty good deal if one was doing extensive travel in Germany, perhaps to be supplemented sometimes by landertickets.

Also, it doesn't say anything about having to pay the intercity "zuschlag", so I'm thinking you don't have to with it?

@ rob... I tell the staff person at the Reisezentrum when I want the Pass to begin. Usually it is to start on that day or the day after The staff person fills in the dates of validity. I use the 10 day/2 month/2nd class Pass. I've never had any problems with the Pass and the staff person in that regard.

"Do they mean you have to pick all 10 travel days right then when you purchase it?"
No. You select the first day of validity - which also determines the final date of validity. You use it on any 10 of those 30 days.

"Also, it doesn't say anything about having to pay the intercity "zuschlag", so I'm thinking you don't have to with it?"
There is no additional fee for IC or ICE trains. Reservations are not required, but you can pay for them if you wish.

"...the price of 520 euros for two people to have 10 travel days within a month seems like a pretty good deal if one was doing extensive travel in Germany, perhaps to be supplemented sometimes by landertickets."

That's €26/person/day, indeed not a bad price if you have long travel legs. It's probably unusual to pack that many long trips into a month, however. The recent addition of GRP destinations like Venice, Brussels, and other foreign destinations certainly makes the 7-10-day GR passes more attractive than they were before, when it was confined to Germany alone (and Salzburg.)

Laendertickets provide some advantages shared with the rail pass - and some additional ones as well. It's a day pass - so you can use it for multiple (and spontaneous) trips, much like a railpass. The daily cost of Laendertickets is about half the cost of one day on a GRP. Laender tickets don't need to be pre-purchased - so there's no need to refund them and no loss of use or fee if you change your plans. (Not so with the GRP - you cannot get a refund for unused days, only for the whole pass, and a 10-day-pass refund costs about €75.) The other advantage of the Laendertickets is that you can use them on local bus, tram and subway transport within most cities (not possible with the GRP.)