However, apparently, the idea goes back to 1917 Germany.   As Paul Eggert explained in private correspondence,

""" fold=0 and fold=1 is like the longstanding German standard for expressing times as strings, which uses "A" and "B" to distinguish time stamps that would otherwise be ambiguous.  If I understand things correctly, normally fold=0, but you can have fold=1 when time stamps are repeated.  For more about the German standard, you can start with the Wikipedia description here: [2] """

[2]: https://de.wikipedia.org/wiki/Sommerzeit#Offizielle_Regelung_der_Zeitumstell...

My favorite part of that Wikipedia article is this sentence: "Sie darf nicht verwechselt werden mit der international üblichen Abkürzung der Zeitzonen, nach der 2 Uhr MESZ als 0200B und 2 Uhr MEZ als 0200A bezeichnet werden." Which is to say (IIUC) that the international standard is that the first occurrence is B and the second is A, exactly the opposite of the German system. O Weh! ijs