This is somewhat related to the Temporal Associations, described in MRL4 here, and temporal alignment, described in this SE question.
To specifically answer your questions:
However, if UserA broadcasts that TX at 8:00PM, is there any record of that?
The broadcast of the transaction isn't stored in the blockchain, but its time of inclusion into the blockchain is (the timestamp of the block). However, someone sniffing the network could record the time of all transaction broadcasts.
Assuming there is a record, and assuming the analyst sees it is currently 8:01PM, could analysts determine that the most recently created output has a high(er) probability that UserA was the authentic sender?
Theoretically, yes. However, this is only problematic under the following circumstances. 1- you just received your monero, so you only own recent outputs. 2) you only own outputs in 1 block.
Issue number 2 will be somewhat fixed with RingCT because denominations will not be necessary so there will no need to stack many outputs into a transaction - so no alignment.
Issue number 1 can only be dealt with (using the current Monero client) by letting your monero "bake" in the blockchain. Future developments could include custom ring partner (mixin, fake output) selection, such that the user could decide which ring partner collection (created at random using probability density functions of different distributions) will work best for the outputs owned by the user.