You know the feeling.
You just invented time-travel and you get overly eager and use the first test-run to travel back to the viking age - 'cause, VIKINGS!
Then you get there and - well - they don't exactly appreciate someone showing up in a "land ship", so they bust up your "ship". And like that, you're stuck.
Time goes on and you use your knowledge of, well, everything to get ahead and you somehow make your way to being the clan leader, then the leader of the local group of clans, and so on until you eventually find yourself uniting all the clans of the entire land into one big alliance - one country. All of this while you're still trying to get the materials to repair your time-travel device.
A few years go by and you eventually find yourself only missing one last material to get your time-travel device working again; though not one easy to find. You have however heard of the material being present in a temple in a city in a far away land - the place that will later be known as Istanbul. You invite all the experienced merchants and a few fighters, and start the journey.
A large group of people are traveling with you to the place known as Constantinople in this time. This your first chance to get back to your own time - and nothing will distract you.
As you get to Constantinople you start sending out feelers for the last material you're missing; and it doesn't take long for word to get back to you that what you're looking for is available in one of the biggest temples in the area - Hagia Sophia. This is your chance.
You have brought everything you need to assemble your device, add the final ingredient, and travel back to your time. You are on way to meeting with the holy man who has the thing you need. You show up - and he's late. He shows up - and he has it!
The deal is straight forward - something you're not used to with your usual viking deals. You pay him and you finally have all you need. You take your bag with everything and head towards an empty area of the temple. You are on your way back - back to where you belong.
You stop. You look around at all these people who have no clue what is going on. These people who don't mean anything in the grand scheme of things - and who won't show up in the history books. As you think about it - none of this, none your time here, or anything else have been documented; this primarily due to documenting anything is not yet something people does (at least not vikings). You know what you have to do. You grab your knife; you kneel down; you start carving your message into the floor of the temple - a message that will echo throughout time...
"HALFDAN WAS HERE!"