Kiddush between 6 and 7
In Chabad today, for instance, the custom is observed in a most contrived way. The term "6 oclock" is taken literally as the hour we call 6 (or 7 if your locality has daylight savings time). If it gets dark at 4, the forbidden hour is still 6 to 7. If it gets dark at 7:30, the custom does not apply. In Chabad, they do not make kiddush early, and they always use red wine (unless vodka is available). Hence, in Brooklyn, they rush home in December to make kiddush before six oclock, or they delay until after seven. Not only that, but this practice comes from the top, I am told. Among the Chabad Rebbe's comments are an adherence to the literal 6 oclock, though no one seems certain if he held the custom at all. No one remembers seeing him make kiddush on Friday night, and his comments are conditional ("for those who do not make kiddush between 6 and 7").
One thing is clear, though. The custom, as practiced by Chabad today, is NOT sourced on the Magen Avraham. The MA is a comment on why one is allowed to make kiddush early. This is not Chabad minhag. The MA is talking about the first hour of the night. The Chabad custom is 6 oclock on the clock, whether it is broad daylight or sometime into the night. Chabad Rebbes have held against using white wine between 6 and 7. If one cannot make kiddush by 6, Chabadniks will wait until after 7, a circumstance not mentioned in the MA (though he alludes to an earlier practice from the Yerushalmi)..
So we are left with questions which render the matter almost absurd.
What does Mars have to do with kiddush? Why is Mars negative? Does it go back to its Greco-Roman "war god" mythology? Where is this mentioned in sifrei kabalah?
Now that we know about time zones, do we adjust the hour for the time zone? As we proceed west in a time zone, should we start at 6:15, 6:30, 6:45 based on where we are in the timezone? Otherwise, isn't the hour 6:00 entirely arbitrary?
It seems that the only effects possible from remote planets are by reflected light and gravity propogation. (Electromagnetism is too weak for these planets at their distance). Both reach Earth from the source at the speed of light. Should we not adjust the minutes based on the distance (in light-minutes) of these planets from us at the time of kiddush? Note that these will vary quite a bit, depending on where we are in our respective orbits. Thus, if we are on the opposite side of the sun from Mars, we would have approximately 8 minutes for the sun and another 15 minutes for Mars to affect us. should the lead edge of time zone start at 6:23 in that case?
What basis do we have to make kiddush after 7? What is gained now that the astrology is not applicable after 7? The MA brings a Yerushalmi that says that they made kiddush an hour or two later in the night, obviously connecting it. But on what basis?
Note that we might say that the time is arbitrary so we can call for 6:00 whenever we designate it, but the custom is already adjusting for daylight savings time, an arbitrary distinction. Therefore, we must say that the custom intends something reality based, not just an arbitrary point in time that we call 6 oclock.
In the end, I cannot find anything to define this custom. It seems to be an arbitrary time loosely attached to some arbitrary astrology, that has no wide source in Jewish law or history before the Magen Avraham (and his sources, the Maharil and Tikunei Shabas and a single memra in Yerushalmi). Time to dump it into the "it's secrets of Torah" bucket.