I am not sure what "timestamp" you are referring to, and what you mean by "the details for a message."
Every message has two date/time values; the "Date:" header in the message which records the date/time the message was sent according to the sender's point of view, and the IMAP "internaldate" which is stored in mailbox metadata which records the date/time the message was received according to the receiver's (typically the IMAP server machine) point of view. I am guessing that by "the details for a message" you mean a view that looks at the message header. If so, what you see after the Date: line is the sender's date and time. I am also guessing that your client shows the "internaldate" in "the listing of all messages in a folder". Many, but not all, clients do that. If the internaldate is displaying in UTC (the time formerly known as GMT), that means one of two things. Either the client is converting the internaldate to UTC, or the IMAP server system's default time zone is in UTC. The IMAP server has nothing to do with setting the default time zone. This is done by whatever program receives and delivers the mail. On many systems, this would be sendmail and /bin/mail; but you have to check your system to see how mail is actually configured. If you are using some other mail delivery program to deliver mail, you have to check to see how that programs works. For example, the dmail and tmail programs use whatever the host operating system has as its default time. In other words, this is not an IMAP server issue; it is a mail delivery issue, and you need to continue your investigation with however mail is delivered.
