Each time you certify effort, you do so for a specific period of performance. The periods of performance correspond to the fiscal year pay periods as they are earned.
The performance period will be semi-annual. All performance periods will reflect 13 pay periods (1-13 and 14-26). Fiscal year 2017 will have 27 pay periods (1-14 and 15-27).
The time during which you certify effort is called the certification window. Each certification window begins about a month after its corresponding period of performance.
Each time a semi-annual certification window begins, you have 21 days to certify the effort for the corresponding period of performance.