[TheTaytay]

Doesn’t it guarantee consistency from the time the transaction started (assuming read committed isolation)? It guarantees you won’t see something “later” than when your transaction began.

https://www.postgresql.org/docs/current/transaction-iso.html

I’m likely misunderstanding what you mean by time.

[Spixel_]

Read committed (which is the default), doesn't guarantee that. See "Nonrepeatable Read" and "Phantom Read" which are both possible in your documentation page.