I think we've already been over this...but...I've actually bothered to look it up now.(noticing today how quickly it got dark wot prompted it)
So, perhaps surprisingly (? it not being the Solstice - shortest day) Johnson seems to have chosen a date for the General Election which falls within the 5 day period of earliest sunsets (15:51pm). Although, because of the sunrise differences 12/12/19 has, in total 5 minutes more light than the days around the Solstice.
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So, well done Johnson...you've actually determined GE day will, pretty much, feel like the shortest day to many folk. Heard some ERO or summat from the IoW (likely to have one of the latest sunset times) say that he reckoned that there'd be approx 7.5hrs of darkness for polling.
Anyway...nerding over.