The timechart command in Splunk requires the _time field to be present in the dataset because it uses time as the primary axis for charting data. The _time field represents the time of events and is essential for commands that generate visualizations based on time, such as timechart. This command groups the events into time intervals and performs statistical functions on those time intervals. Without the _time field, the timechart command will not function properly.
Reference:
Splunk Docs - timechart command