Earlier quoted context omitted.
You can find better references in the czech version of the wiki article https://cs.wikipedia.org/wiki/Karlův_most Basically, it is just a hypothesis by some astronomist. Contemporary chronicler recorded just the year that the construction was started
My Czech is not the best, if I'm honest... So, your reading is that its a hyposthesis by some astronomer. And that someone chronicled the year construction started. If you are able to provide sources for those, that would be interesting! Cheers
https://www.academia.edu/3093773/Založení_Karlova_mostu_a_ko...
(alert: scanned and turned 90 degrees, my neck hurts)
On page 202 it describes founding of the bridge. The author puts it in context of gathering the remnants of St. Vitus (that Charles was obsessed about and his statue is present on one of towers at the end of the bridge) and highlights that the conjunction (opposition) of Saturn and the Sun happened on 1357, 9th day of 7th month.
He further investigates whether the way Charles or people of his age would write the date is the same as ours and concludes that it is likely - arab numerals, year, day, month then time, 60-minute hours are all documented in his court. Founding of the bridge would have to include a mass so it could not take place in the afternoon, which 5:31 AM isn't. Indeed different ways of expressing time used at the time are all consistent with that - except the "old bohemian" way to express time because that would be before sunrise.
If they did express it the same way as we do then at 5:31 AM the constellation of Lion (symbol of Czech kings) was in ascendent (just above horizon, important astrological place).
Additionally, a sign (which was dated to 1640 but could have been a replacement of an older one) on one of the towers at the end of the bridge contains a palindrome poem (in Latin).
Another reason why it could be true comes for the author from the fact that Charles and his people would have known and read Dante Alighieri's Vita nuova where similar numerological symmetry plays a major role. And the Prague Astronomical Clock contains a completely extra gear that serves no purpose but the counts of its tooth is also a palindromatic sequence.