Hello RikFoj,
It sounds like you are needing a copy of your iMessage transcripts that include time and date stamps.
Your best resource to receive a complete log of messages with time stamps and dates would be to check with you carrier.
However, if the messages only exist on your Mac, you can hover your pointer over any conversation bubble and it will show you the date and time it was sent or received at. You can screen shot this information one conversation bubble at a time by selecting the key combination: Shift, Command, and 5.
If you have an iPhone that you can view these messages on, you can swipe left on the conversation and see a conversation log that includes date and time stamps.
Please feel free to follow up in the event you feel you need additional guidance. We appreciate you reaching out to Apple Support Communities.
Be well!