The software is installed locally (runs on macOS and Windows), but your files are stored on Autodesk's servers. You have to sign in to use it. Program startup takes ages. But the features are great.
If you don't need fancy features, have a look at FreeCAD as well.
Clocks do drift. Seconds a week is definitely possible. I think there are varying quality of internal clocks in electronic devices, and the cheaper the quality the more drift there is. I think small cheap microcontrollers can drift seconds per day.
I have an extremely cheap and extremely low power automatic cat feed - it’s been on 2 D batteries for 18 months. I just reset it after it had drifted 19 minutes, so about 1 minute a month, or 15 seconds a week!
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