If all events from this source have eventStartTime you can setup a props.conf setting for that source/sourcetype that tells splunk what timestamp to use when assigning the _time value.
Based on the event you provided , and assuming that your events are not multi-lined, you could add this to your indexers props.conf.
[source::/opt/log/yourLog]
TIME_FORMAT=%Y-%m-%d %H:%M:%S.%3N
TIME_PREFIX=eventStartTime=
SHOULD_LINEMERGE = false
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