1. A computer-implemented system for processing time series data that is indicative of a data generation activity occurring over a period of time, comprising: a processor; a computer-readable storage medium containing instructions configured to cause the processor to perform operations including: storing characteristics that define a time series model in a model specification hierarchical data structure that includes a configuration for storing time series model characteristics in an Extensible Markup Language (XML) format; storing characteristics that define a fitted time series model in a fitted model hierarchical data structure that includes a configuration for storing fitted time series model characteristics in an Extensible Markup Language (XML) format, wherein the fitted time series model characteristics include characteristics associated with a forecast function that describes how to forecast the time series data, and wherein the fitted time series model characteristics are defined through an application of the time series model to the time series data and using the stored fitted time series model characteristics to provide forecasts of the time series data.