What are Elite Rollover Nights?
In order to qualify for IHG Rewards Club Platinum status, you must stay 50 nights at IHG properties in one calendar year. Once you've stayed 50 nights in a calendar year, there is no further status level to work towards, so any nights you stay above and beyond 50 are 'rolled over' to the next year, thus giving you a head start on re-qualifying for Platinum status next year.
As an example, if you were to stay 60 nights in 2014, 10 nights would be rolled over to 2015, counting towards your platinum requalification for 2016.

I've qualified for Platinum through the Fast Track offer of 40 nights over 4 brands - will my extra nights now roll over?
Elite rollover nights only kick in once you pass the 'standard' requirement of 50 nights in a year. Whilst 45 nights, for example, would be more than you'd need for Fast Track Platinum, you'd have none rollover.

I've qualified for Platinum through elite qualifying points earned, or through a status match, rather than nights stayed. Will my nights roll over from now, or do I still have to reach 50?
Regardless of your status through points earned or status matched from a competitor, unfortunately you will still need to reach 50 nights before your subsequent nights will roll over. This probably won't work in your favour if you spend a small fortune in expensive IHG properties and thus earn lots of points!

Will this year's rollover nights count towards next year's rollover?
Elite nights will not roll over more than once, and thus will not count towards future years.
If you stayed 60 nights in 2014, thus rolling 10 over to 2015, you would need to reach a total of 60 nights (50 excluding previous rollover) in 2015 before your subsequent nights would begin to roll over.
Put simply, you need to physically stay 50 nights in IHG hotels each year to start counting toward the next - 45 nights plus 10 rolled over from the previous year would still leave you with zero rolled over to next year.

My elite year has reset; yet My Account shows 0 rollover nights when I believe I had some due!
This seems to be a common issue, though elite rollover has only been around since 2013, so knowledge is a little restricted at present.
At first it's probably best to wait a week or so and see if they eventually update, as they often do so eventually, it's just stereotypical of IHG's dodgy IT.
Some also report that, when your first stay of the year tracks, the rollover nights will appear with it.
If you don't seem to be getting anywhere though, use the platinum contact form on the IHG website to voice your concern. Let them know how many nights you believe should have been rolled over (see the next FAQ if you're not sure) and they should be able to sort it out if you are indeed eligible, even if it means a manual credit.

I've forgotten how many elite qualifying nights I made last year - is there a way to check?
Unfortunately IHG's website does not go back further than 6 months for your stay history, which is fairly useless in this case. However, the IHG mobile site does let you see all your stays; seemingly since the beginning of time. Head to this link and log in with your IHG RC credentials. Below your pending reservations, you should be able to see your previous reservations. Now it's just a manual job of counting the number of nights, including reward nights!