Trillian supports customizable per-message timestamps. The following options are supported:
Show where I last left the conversation. Adds a line to your message window representing the last message you read. If you switch from Trillian to another application and later come back, this will can help aid your eye in knowing where unread messages begin.
Show timestamps for each message. You can uncheck this if you'd rather not see timestamps, and you can also customize the format used for timestamps. Click the edit control and hover your mouse cursor to see a list of formatting options, which will update in the above preview in realtime as you type.
Click the Trillian button in the upper left of your contact list and click Settings:
Click the Chat Windows category and scroll down to Timestamps: