The reporting period for 2017 MIPS is the calendar year.
In the transition year, which is 2017, providers are able to "pick their pace". There are 4 options: not reporting which would result in an automatic 4% penalty, the test pace where a provider submits some data, a 90 day reporting period or a full calendar year.
The 90 day reporting period can be any continuous 90 day period in 2017. You will not be able to collect data using this option after October 1.