Amsterdam Card at Schiphol

I've never used it, but their website gives the information you want.
You can buy it on Schiphol airport. It is not valid for the train into the city, but is on the city (GVB) bus # 69 to station Sloterdijk where you have to change to further transport to your destination.
It works on the 24-hour system, starting with your first use of the card.

Jon, you can answers on the website for the card. :O)

http://www.iamsterdam.com/en/i-am/i-amsterdam-city-card

Can a 2 Day Amsterdam Card be purchased at Schiphol?

Yes:
https://www.iamsterdam.com/en/i-am/i-amsterdam-city-card/how-does-it-work/sales-locations

Is the card good for the train from Schiphol to Centraal?

No:
http://www.iamsterdam.com/en/i-am/i-amsterdam-city-card/what-is-included
"Note: Trains, including the ride from Schiphol (Amsterdam airport) to Amsterdam Central station, are not included."

Does the card calculated by calendar day or "24 hour day"?

By hour from the time of activation: http://www.iamsterdam.com/en/i-am/i-amsterdam-city-card/how-does-it-work

"Once activated, each element remains valid for 24, 48, 72 or 96 hours, depending on your card."

If you are just looking to buy a train ticket from the airport to Centraal station and then back again, it is much easier to buy these online, rather than wait in line when you get to the airport. I just bought my round trip tickets for August and it was easy and cheap. Much easier than the last time I was in Amsterdam in 2014. Link below. Also the tickets are coded for your name and exact dates, so make sure the dates are correct before ordering. They are emailed almost immediately. great system.

http://adaminfo.rgi.ticketbar.eu/en/ticketbar-amsterdam/train-from-schiphol-to-amsterdam-/

Orv

You can get a 2-day transport card that includes the the train to and from the airport. Its called the Amsterdam Travel Ticket and costs 21 EUR vs 12.50 EUR for GVB ticket.
https://en.gvb.nl/tickets
For sale at a couple of airport locations. Click on "points of sale".
IMHO, the Iamsterdam Card is a bit overpriced. Depends on what you want to do. I'd just go alacarte and get the Travel Card.

That's a good point, Sam. As with any pass, you have to be interested in enough of what they cover - and enough time to fit all of your chosen attractions in - to make them worthwhile. That was our problem with the I Amsterdam Card: it didn't cover two of the biggest things on our list, several others were closed for renovation (not a problem anymore, I don't think) and much of what else it included we weren't all that jazzed about. The transport piece would also have been a waste as we prefer to sightsee on foot.

Anyway, that's why we decided to go the à la carte route versus the pass.

Thank you to all who posted. We can now make an informed decision on how to go.
Jon