when you created the index pattern for your data, you picked the primary "time" field (the one the time range picker from the top right is applied to) marked by a little clock icon in the index pattern management - in this screenshot it's timestamp:
It should always mark a field that contains the date the event you logged occurred (all of your documents in the index need to have that field set)
If you don't have that clock icon or it marks the wrong field, please re-create your index pattern and make sure to pick the right field in step 2 of the creation process:
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