Long, long ago in a newsroom far, far away – actually, it was 1999 in Nashua – I squandered many valuable hours arguing with people who insisted that the new millennium wouldn’t begin until the year 2001.

Two decades have gone by but those folks are still around, or maybe their younger siblings have taken up the cause, so it’s time to swat them down once again.

The argument, as you may know, runs like this: The calendar goes directly from 1 B.C. to 1 A.D. (using the old terminology). Since there was no Year Zero, the first decade began on Jan. 1 of Year One, which means it lasted until Dec. 31 of Year 10.

Ergo, say these misbegotten nit-pickers, decades turn over at the conclusion of years which end in zero rather than nine, and the current decade does not end Tuesday night but sticks around through all of 2020.

This argument convinces many people because it sounds math-ish, but it’s a bunch of baloney.

For one thing, the calendar based on Jesus’s birth wasn’t even established until the Year 525 and was an arbitrary guess. Using it as an objective measurement of universal truth is fatuous.

More importantly, decades are cultural constructs rather than astronomical realities. We create them for convenience, and it’s convenient to say that 2020 is part of “the ’20s.”

After all, did you stand up at your 20th birthday party and say “I’m now in the final year of my teens!” Not even millennium-doubters were that silly.

But there’s yet another problem with the whole argument. There was a Year Zero – it just wasn’t a year long.

Let’s ignore the historical realities of Jesus’s birth, which probably occurred around 4 A.D. according to current historical analysis. It also probably occurred in late winter or early spring, when shepherds keep watch over their flocks at night because it’s lambing season, rather than on Dec. 25. That doesn’t really matter.

Imagine you’re standing outside that manger in Bethlehem the day after Jesus was born. What year is it?

It’s not 1 B.C. because it’s not Before Christ. But it’s not 1 A.D. either.

Consider: Your first birthday party marked the end of the first year you had been on the planet, not the start of it.

By that same reasoning, Jan. 1 of Year One didn’t come around until the end of the first year that Jesus had been on the planet, even though this “year” wasn’t 365 days long because we didn’t match up Jesus’s birth with New Year’s Eve.

Ergo, say I, the period of time between Jesus’ birth and Dec. 31 was Year Zero. It’s a unique “year” because it was short but it did exist.

That means the first decade lasted until the end of Year Nine, and Year 10 was the start of a new decade. Just as 2020 is the start of a new decade.

So when people start blathering about “not a decade” and “no year zero” you tell them they’re wrong three times over – wrong historically, wrong culturally and wrong on the whole calendar thing.

Then go find some mistletoe to stand under. It’s that time of year!