Board Thread:Game Discussion/@comment-92.233.114.15-20160122174206/@comment-4939977-20160123120246

Take for example, servicing an Audi R8 V10 Coupe. The servicing time will remain stuck at 2 hours remaining even after more than 2 hours have passed while offline.

So you need to play online at all times (when online, the in-game time will also count down in real time). Another example is Stage 1 upgrades for supercars, which take 30 minutes each, you need to be online if you want those 30 minutes to really count.