Mitigate false positives due to daylight saving time in CloudWatch anomaly dectector
My current understanding is that it is not possible to avoid false positives. At least not if traffic comes from different time zones around the world (as in the context of this question).

AWS operates on UTC times, but this is irrelevant for the anomaly detection. It does play a role if you set timers, but it is not taking into account for the predictions of AWS. In other words, switching to UTC will not make false-positives less likely.

When thinking about it, it is hard to avoid false-positives. If a  time-shift in Europe or the US happens, you could take that  into account and predict the new traffic. But that requires extra domain knowledge. The AWS predictor only sees a  generic time series.

There are other events such as Christmas, which will impact the expected traffic. Again, with extra knowledge you could improve predictions. But the problem is too difficult for the AWS predictor, as it does not have that domain knowledge.

In our concrete case, we still kept the alerts, as they do have value even with the known false-positives. Humans with knowledge about the underlying system, can reject the false-positive. It is not ideal, but it is a trade-off that we are willing to make.

Note that other types of graphs are not susceptible for these types of problems. For example, error rates instead of total number of errors. It is not always possible, but if you can use rates, the system will be more stable in my experience.
AWS allows to set alerts on math expressions such as errors / requests. The advantage is that normalized rates tend to be agnostic of external events.