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Antoine and others:
What did you select for the star positions? Many of the stars are trailed or streaked (curiously not all of them). This could be a result of mechanical tracking or possibly this is handled by stacking software. Everybody does stacking these days in astrophotography, and it's possible to "track" any object in software with any rate in any direction. I assume that there are also tracking modes designed to follow features 'visually' rather than just based on traditional rates of motion. Regardless of the origin of the streaked star images, one could choose the lower end as the latest time since the Moon is moving mostly upward in the image relative to background stars, or the upper end of each streak as the earliest time, or the middle as a best compromise since some of the stars do not show much trailing at all. I would not say that there is a "right answer" here, and I think this is just part of the process of estimating error bars on the UT of the photo.

Frank Reed