According to EA, it took over 6.5 months
to create the new offline gameplay patch for SimCity. Even though offline gameplay patches were published by modders in the weeks after the game's launch, EA claims none of these mods were able to perform actions like communicating with other cities or other regions and that it took a significant amount of time to rewrite the game to allow the simulation to work as intended without being connected to the Internet.
"My team did, however, see a path forward towards Offline, one that would maintain the integrity of the simulation. Lucy once said that Offline wouldn't be possible 'without a significant amount of engineering work,' and she's right. By the time we're finished we will have spent over 6 ½ months working to write and rewrite core parts of the game to get this to work. Even things that seem trivial, like the way that cities are saved and loaded, had to be completely reworked in order to make this feature function correctly."