# time

SBML Level 2 uses the MathML `<csymbol>` element to denote
certain built-in mathematical entities without introducing reserved names
into the model's component identifier namespace. The "time** **
`<csymbol>` represents the current simulation time in a model.
The units of the current time entity is determined from the SBML
*time* units (either as defined in the model, or the default SBML
time units if a model does not redefine the default).

Note that the value of the `<csymbol>` does **not**
represent a symbol to be used to stand for time in the model. The
`<csymbol>` element *itself* stands for time. The following example
attempts to illustrate this:

<math xmlns="http://www.w3.org/1998/Math/MathML"> <apply> <plus/> <ci> x </ci> <csymbol encoding="text" definitionURL="http://www.sbml.org/sbml/symbols/time"> t </csymbol> </apply> </math>

The token `t` in the above fragment **does not** enter into the
SBML component identifier namespace. The token `t` is mostly a
convenience for human readers—the string inside the
`<csymbol>` could have been almost anything, because it is
essentially ignored by MathML parsers and SBML. Some MathML and SBML
processors will take note of the token and use it when presenting the
mathematical formula to users, but the token used has no impact on the
interpretation of the model.

In SBML, the encoding attribute of `<csymbol>`
must be set to "`text`".