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".