Most of the values calculated in OpenFisca, such as an income tax, or a housing allowance, can change over time.
In simulations, parameters and variables, OpenFisca handles time via periods and instants.
In OpenFisca context, a period consists of:
a starting instant (the atomic unit is a day, so instants are day dates),
a unit (day, month or year),
a quantity of that unit.
Let’s look at some examples:
here is an instant: the 15th June 2015,
and here are some periods: July 2015 is a month starting on its 1st day, 2015 is a year starting on its 1st day,
we can also define a period of several months, July and August 2015, or a period overlapping different months, one month starting on the 15th of July.
The smallest unit for OpenFisca periods is the day.
The largest OpenFisca period is the eternity, which is used for variables that are constant over time, e.g. a date of birth.