Plans have obviously have to be different.
I see this is SQL Server and it will cache plan for an exact query you run. By moving date time clause to join condition you force a new plan generation.
You should be able to get same results by forcing plan recompilation
I looked at my query again and I found out that I had an extra clause NOT EXISTS which was causing difference in speed and plans, omitting not EXISTS didn't make difference in speed in both cases.
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