How to get the duration between 'Suspended - Payroll Eligible' to 'Active - Payroll Eligible'.

Ex: There is one absence plan called Vacation, for this accrual method is incremental and repeating period is monthly. here we have a requirement that, if employee assignment status is 'Suspended - Payroll eligible' then accruals can't be credited and once he becomes 'Active - Payroll Eligible' then again, he will starts accruing balance.
If the duration is 15days then, how to get the duration between 'Suspended - Payroll Eligible' to 'Active - Payroll Eligible' to prorate the accrual balance in that particular period.