I mentioned that I had thought I learned that on the Maharil’s stance on tzistzis, but I wasn’t sure. Thanks for the source. L’maaseh, though, nobody holds by it. Everyone wears some form of tzistzis before marriage.
The 6-7 thing always went by the goyishe clock. I don’t think a custom going by the goyishe clock makes any sense to begin with, so I don’t know that it makes inherently less sense not to index for DST, the times are all arbitrary anyway.