Daas Yochid,
The “standard of the day” I am referring to is not based on the non-Jews or the non-frum Jews.
Daas Yehudis means the the norms of frum women in a frum community. Furthermore, there are also some levels of tznius that are governed by takanos d’Rabbanan; but the covering of arms is Daas Yehudis. So if one would live in a frum community where the frum women will bare their arms it would be mutar.
A minimal hair-covering according to most poskim is d’oraisa and it certainly wouldn’t change based on the communities norms (but the more extensive hair coverings we have in our communities are Daas Yehudis).