The "offset" attribute is only available in SBML Level 2 Version 1. This attribute is not present in SBML Level 2 Version 2 or above. When producing SBML models using these later specifications, modelers and software tools need to account for units with offsets explicitly. The SBML specification document offers a number of suggestions for how to achieve this. LibSBML methods such as this one related to "offset" are retained for compatibility with earlier versions of SBML Level 2, but their use is strongly discouraged.
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