I indexed a dataset with no timestamp (TZ=local) in a locally installed Splunk, and did the same thing. I guess timezone should match among timestamp in data, Splunk/OS and Browser I am running with. Still timechart behaves the same. Depending on the span, timechart snaps to 00:00 or some offsets. Specifically, with the latest and earliest is set to a day(-1d@d to @d), timechart starts from 0:00 only in case of span=1h,3h,7h,21h. (for a day with span more than a few hours does not seem to have much meaning, but timechart behaves diffetently depending on the combination of span and time range.
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