2 Terminology

As a model advances in discrete timesteps the time manager keeps track of the date and time at each endpoint of the current timestep. We begin with this section on terminology to clarify our use of common words like ``date'' and ``time.''

date

The term ``date'' is used to refer to an instant in time. It consists of year, month, day of month, and time of day components. The time of day is expressed in UTC. A date specification is incomplete without it's associated calendar.

time

The term ``time'' is used in the sense of ``simulation time'' and expresses an elapsed time since a reference date. These time values follow the convention for time coordinates supported by the COARDS [2] and CF [3] metadata conventions.

time of day

Time of day refers to the elapsed time since midnight of the current day. We express time of day in UTC unless explicitly stated otherwise. In common usage the term ``time'' refers to ``time of day.'' In this document we will use ``time of day'' explicitly when that is what we mean.

start date

The start date of a simulation is the date assigned to the initial conditions.

reference date

The reference date of a simulation is an arbitrary date that corresponds to the origin of the time coordinate. Often this date corresponds to the start date. But it may be different because providing a reference date that is common to a set of simulations that may have different start dates allows them to use the same time coordinate.

current date

A simulation advances by timesteps. At any point during a simulation the current date is taken to be the date at the end of the current timestep.

start time

The start time of a simulation is the elapsed time from the reference date to the start date.

current time

The current time of a simulation is the elapsed time from the reference date to the current date.

calendar day

The day number in the calendar year. January 1 is calendar day 1. Calendar day may be expressed in a floating point format consisting of the integer day number plus the time of day (UTC) represented as a fractional day. For example assuming a Gregorian calendar:

Date	Calendar day
10 January 2000, 6Z	10.25
31 December 2000, 18Z	366.75

restart date

The restart date of a simulation is the date of the data in the restart files from which the simulation is continuing.