User blog comment:IamJakuhoRaikoben/Chapter 468: Memory of the Stars, Review./@comment-7418318-20160109195533/@comment-24.177.95.45-20160109212141

I believe you need a lunar eclipse to open the gate so the family might have been pick the era but the month and specific year was out of Layla's control. She probably only had one night to open the gate or wait for the next Lunar eclipse.