While 13 years has passed since the manga started, I don't think 13 years has actually passed in the manga storyline. Of course, time has progressed for the OMG crew, just at a different rate. Using K1 as a reference: K1 has graduated college and has started working for Chihiro since the manga began. This would suggest that roughly 3-4 years has passed. With the two older goddesses, it is harder to tell based on their appearances how old they are. Skuld definitely seems older, at least based on her actions.
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