JW's calculate Nisan 14 to be 14 days from when they estimate the crescent of the said new moon will be visible over Jerusalem. Needless to say they are often wrong. A few years ago they said the new moon would NOT be visible on this particular day but in fact it was clearly visible both in Jerusalem and also the UK.
The Jews calculate Passover to be on Nisan 15 which they calculate as 14 days from the astronomical new moon which is why Passover almost always coincides with the full moon and the JW Memorial usually does not. This year the full moon is about 06:30 tomorrow (Thursday) but still on Nisan 15.
The Israelites left Egypt on the morning after the Passover, it was still Nisan 14 of course by JW reckoning. In view this I asked an elder to explain the meaning of Numbers 33:3. He told me I think too much which is not good!
For anyone interested it's worth drawing out a timeline of events for Nisan 14/15 bearing in mind the crazy explanation for "between the two evenings". Then ask how long it would take you to kill a sheep, bleed it, skin and gut it and then roast it whole over an open fire. I've never tried it but extrapolating from the time it takes Sis England to cook a leg of lamb in a modern insulated fan oven.........
George