The user/date time may or may not be available. It will take some digging.
Sage stores the last user, date and time that updated a record internally. You will need to read the database directly in FixIT or MsSql Studio. Each table will contain the Audit User, Audit Date and Audit Time. This is not a log of all users that added/changed the post, only the last user to make the change. It also assumes the user was logged in as them self and not another user.
Your IT department may have a transaction log created by MsSql server. Much harder to dig through to find the information.

It is stored in the GLPOST table in the AUDTUSER, AUDTDATE and AUDTTIME fields. You can also look in the GLJED (GL journal entry detail) table to see who created that entry - someone else may have posted it.
Actually that info is stored in every table, so knock yourself out.