I don't think we should store this information directly on the model object. If you need this information, which I can understand why you want to do it that way, I'd recommend just calculating it on the fly rather than storing it. You simply have to iterate over the comments and find the last updated at time (often the last one in the list so you may just be able to grab the last one).

Reasoning:
1. This style of syncing allows you to sync one thing at a time, aka when you hit that test run's test case
2. If you're only updating one at a time, then you won't suffer from having to calculate it on the fly from maybe 10~? elements tops.
3. Keeps syncing logic out of the model object, which should remain as pure as possible.