The CGM is known to be a complex environment with a litany of evolutionary processes occurring at once. In this work, I use a subbox from the IllustrisTNG simulation to study the short-term (<1-2 Gyr) evolution of CGM gas around Milky Way progenitors at z~1. This subbox has a very high time resolution (~3 Myr between snapshots) allowing for detailed time-series analysis of the effects of evolutionary processes in the CGM in a new way. I find that a majority of the CGM at z=1 stays in the CGM for the following ~2 Gyr, indicating that a significant amount of gas processing occurs before accretion onto the galaxy. On this timescale, CGM gas evolves into a state composed primarily of a cold, dense, low-entropy phase at small radius and a hotter, more diffuse, high-entropy phase at large radius. This evolution occurs for all CGM gas regardless of its initial location in the halo and is likely driven by star formation and satellite mergers. On timescales <500 Myr, CGM gas "forgets" its kinematic history and generally experiences large changes in velocity and temperature on a particle by particle basis, highlighting the complex nature of CGM gas processing that causes it to evolve into its two-phase state. By generating mock observables from the simulations, I find that gas in intermediate ion phases like CIV does not stay in that phase for very long, and redistributes itself to both colder (MgII) and hotter (OVI) ion phases, which themselves persist for over 1 Gyr.