lua spec has os.date if this was supported we could do os.date("%a", os.time-(24*60*60)) to return the string Wed (which is yesterday when today is Thursday).
In addition, it would give access to the ISO week number via os.date("%V"). The week number currently available is not useful for anyone outside the US.
We are still waiting for this function.
I have a watch face which displays UTC date and requires about 20 lines of code for each timezone. This is not practical which means I can't share the face with others.