# Years, periods, and time slices¶

This page describes how time-related concepts are represented in the MESSAGEix framework and stored in ix modeling platform.

## Years and periods¶

The year set—in a message_ix.Scenario and in the MESSAGEix model formulation—is used to index parameter dimensions with the names “year”, “year_act”, “year_vtg”, etc. Elements in this set are representative years within a period of time. MESSAGEix treats periods as contiguous, but year elements need not be consecutive. The representative year is always the final year of the corresponding period.

Example 1

In a Scenario with consecutive year elements [1984, 1985, 1986, …], the element 1985 refers to the period from 1985-01-01 to 1985-12-31, inclusive.

Example 2

In a Scenario with non-consecutive year elements [1000, 1010, 1020, …]:

• the element 1000 refers to a period that ends 1000-12-31, i.e. at the end of the calendar year 1000.

• the element 1010 refers to a period that ends 1010-12-31, i.e. at the end of the calendar year 1010.

• the element 1010 therefore refers to the period from 1001-01-01 to 1010-12-31, inclusive.

The parameter duration_period stores the duration of each period, measured in years, indexed by elements from the year set. MESSAGEix only supports year set elements that can be represented as integers, and therefore periods with durations that are a whole number of years.

Other parameters may be aligned to the start or end of periods.

Example 3

A Scenario has year elements [1000, 1010, 1020, 1030, …], and a technology parameterized with technical_lifetime value of 20 years for the key year_vtg=1010.

• Capacity associated with this technology is constructed at the beginning of the period denoted by 1010, i.e. as of 1001-01-01.

• By the end of this period, 1010-12-31, this technology has operated for 10 years.

• The next period, labelled 1020, ends in 1020-12-31. By this date, the technology has operated for 20 years, equal to its technical lifetime.

• The following period, labelled 1030, begins on 1021-01-01. The capacity created in year_vtg=1010 is beyond its technical lifetime, and unavailable in this period.

## Time slices¶

The time set is used to index parameter dimensions with the names “time”, “time_origin”, “time_dest”, etc. These are variously referred to as “(sub-annual) time slices”, “time steps”, or other names. Elements in this set are labels for portions of a single year. The special value 'year' represents the entire year.

Since a year element refers to the representative, final year within a period, using year and time together denotes a portion of that specific year.

Example 4

In a Scenario with year elements [2000, 2002, 2004] and time elements [summer, winter]:

• The year element 2002 refers to the period from 2001-01-01 to 2002-12-31 inclusive, which has a duration_period of 2 years.

• The time element ‘summer’, used alone, refers to a portion of any year.

• In a MESSAGEix parameter indexed by (year, time, …), values with the key (2002, ‘summer’, …) refer to the ‘summer’ portion of the final year (2002-01-01 to 2002-12-31) within the entire period (2001-01-01 to 2002-12-31) denoted by 2002.