Nowhere in WT literature is this piece of nonsense spelled out in a way one can understand. There are a number of vague references to three and half years, based on strange interpretation of scripture, one of the Minor prophets Micah or Malachi is invoked, about the "Messenger coming to the Temple" or somesuch.
As all calculations for this are predicated on an acceptance of their laughable 1914 dateing, the whole thing is a nonsense anyway.
All in all, this is the Doctrinal Achilles heel for the WT/GB/JW's as there simply is no satisfactory scriptural reason to believe this happened, a 1919 "choosing" and appointment, and certainly no logical reason.
Get your local Elders to prove it to you, and I mean prove it, not fob you off. If they cannot, there is a simple formula you should show them :
No 1914= no 1919= no FDS= the GB are self appointed frauds.