Does the Elite qualification requirement of 20,000 miles need to be in a calendar year (Jan 1 to Dec 31), or is it 20k miles over any consecutive 12 months?

As of today my qualifying miles is listed at 11,000 - but I haven't flown at all this year (during Jan 2017)
The total of 11k qualifying miles I have seems to be for a few flights I made this fall... leading me to believe that the requirement is for any 12 consecutive month period.