So I realized that Shanks jolly roger could be used to track his scar and checked film red and tried to make a full timeline of Shanks and narrow down when he got his scar.

39 years ago -Gol D. Roger and his crew became the first to reach Lodestar Island: the island at the end of the Log Pose. Shanks was born this very year. As was Buggy.

38 years ago: God Valley incident, a baby (Shanks age 1) was among the treasures Roger took.We know next to nothing about the next decade when it comes to Shanks.

28 years ago - Gol D. Roger discovers he has a fatal disease. Preparing for what would be his greatest journey, he enters the Grand Line and recruits Crocus as ship doctor. Nico Olvia leaves Ohara and her daughter Robin to embark on a Poneglyph expedition.

27 years ago - The Roger Pirates face off against pirate Shiki in the battle of Edd War. Roger escapes, while Shiki suffers massive losses and gets a steering wheel implanted into his head. Shanks does NOT have a scar.

26 years ago - The Roger Pirates and The Whitebeard Pirates fight for 3 days and 3 nights. But make peace on the fourth day. As seen in Oden's flashback Shanks has no scar.Kozuki Oden joins Roger Pirates. Inuarashi and Nekomamushi tag along once again.

Roger arrives in Skypiea and makes Oden engrave the message near the Poneglyph.

Shyarly predicts the Mermaid Princess will be born in 10 years.

Gol D. Roger is shown the Road Poneglyph residing on Zou.

25 years ago: (Shanks age 14)

The Roger Pirates complete their journey to Laugh Tale. Having conquered the entire Grand Line, Gold Roger becomes known as the Pirate King. Shortly afterwards, Roger disbands his pirate crew, allowing them to return to peaceful lives of their choosing. Kozuki Oden dreams of opening the borders of Wano Country in 20 years time. Shanks still has no scar.

24 Years Ago - Roger dies. Shanks begins his journey as a pirate captain by asking Buggy to join his crew. He has no scar.

22 years ago - Shanks meets Yasopp who is implied to be his first crewmate (WE DO NOT SEE HIS FACE)

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19 years ago - Uta meets Shanks who now has been scarred by Blackbeard.

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And that's it. That's the timeline of Shanks life until the earliest point we have seen him with his scar.

Leaving only a only 5 to 3 year period he could have been scarred. So let's narrow it down more with a little guess work. If we go off the idea Yasopp is his first crewmate that leaves two periods in time he got scarred.

One - He got it before meeting Yasopp. During the two year gap between Roger's Death and him meeting the guy. This seems unlikely. Why would Teach be in the East Blue? I don't think a crewless rookie Shanks was heading into the new world to fight Yonko. Yet we know he gets it early on enough to have it be part of his Jollyroger. Do we have any reason to think Whitebeard would have men in the East? Teach was actively trying to not stand out so he likely wasn't going traveling alone.

Two - He got it very soon after meeting Yasopp. This feels possible. Perhaps he formed his crew two years after Roger died. Went into the New World. Something went down and he was scarred. Perhaps leading to him thinking he wasn't ready and he returned to the East Blue and met Uta. Shanks has three years between meeting Yasopp and Uta and Luffy has gone from Arlong to Kaido in that time. It's possible Shanks made it all the way to the New World and then came back to the East Blue? Perhaps a different way to view it. Shanks learned something while fighting Teach and that's why he headed back to go after the Nika fruit? So that's why he returned to the East. This also fits since it'd be early enough in his time at sea to add the scar to his Jollyroger.