You can see these timestamps when you look at details of a transaction on mempool.space. If you have a transaction ID, enter it in the search box on mempool.space to dig up the specific transaction details.

I don't know exactly when mempool.space started to record the timestamp when a transaction was first seen by mempool.space and thus entered their mempool waiting to be confirmed in a block.

It's quite possible that this timestamp is not visible for transactions older than some months or so. There is maybe some notes on this in the changelog of mempool.space.