SQL Server DIFFERENTIAL BACKUP question? Simple question

At what point is a differential backup considered transactionally consistent?

Is it the creation date of the DIFF, or the time at which it completes?

That is if I restore it what transactions can I expect to have been committed? Those that completed before the time the DIFF started, or those that completed during the DIFF, that is the finished time?
Generally speaking a differential works very similarly to a full backup, except that only the changed extents in the DCM are backed up. Differential backups still contain a portion of the log needed to be transactionally consistent. This will differ for every environment and is not "known" at the time of backup. You can use the RESTORE HEADERONLY of the differential to check LSNs that are included and covered. When you think the transaction finished and when SQL Server wrote the logs that may or may not be flushed to disk when the checkpoint ran before the differential backup may or may not be included.