There is a carbon-dating aspect to chronicling the fervor of a Giants fan.

It is not so much how intense or how loyal or how passionate. Often, it is how long.

For a franchise on the cusp of celebrating its 100th season, how long can be very long.

Les Grodnick reports his “how long’’ like this: “The Y.A. Tittle days at Yankee Stadium.’’

That would be the early 1960s. Grodnick has been attending Giants games for more than 60 years. In 1983, he relocated from New Jersey to Miami, so for the past 40 years, Grodnick has been on a road trip every time he sees his team play at home. With the cost of his one season ticket up in section 310 ($1,300, including the parking) at MetLife Stadium, plus his airfare, hotel and car rental, $3,000 is a modest estimate of how much Grodnick spends per year nowadays to keep up an almost sacred family tradition of watching the Giants play football in person.

“I do the best I can with it, it’s manageable,’’ Grodnick said. “In the earlier years, when my dad was living in Florham Park, it was no big deal. I stayed in his house. He died in 1998. After ’98, I was really on my own.

“It is what it is. I don’t complain. I like going.’’

He likes going, but he will not go to night games, and that aversion was the impetus for his most recent correspondence with John Mara, the Giants’ co-owner.

Yes, correspondence. Back and forth.

And the old-school U.S. Mail exchange allowed Grodnick to believe the man in charge cared about him.

In May, after the NFL schedule was announced, Grodnick did what he has done fewer than a handful of times in his six decades rooting for — and purchasing tickets to see — the Giants.

He took pen to paper — no email for him — and reached out to Mara, eager to share his concerns about how the league treated their favorite team. First, Grodnick praised Mara for the strong stance he took against the NFL’s newly hatched plan to flex Thursday night games, lauding Mara as an owner who cares about “the paying customer.’’

Next, Grodnick launched into a rant about the league scheduling the Giants for three night home games, including a Dec. 11 “Monday Night Football’’ meeting with the Packers in the depths of winter, which Grodnick terms “an atrocity.’’

Grodnick’s ire was directed not at Mara but at the schedule-makers, and he implored “the owner of the flagship franchise in the NFL’’ to find out who was responsible and to have them explain why they are not “concerned for the paying customers who go to these games.’’

Grodnick received a one-page, typed response, dated May 30.

“I certainly understand your disdain for night games,” Mara wrote. “I don’t like them either!’’

Mara also pointed out: “If your team is playing most of its games at 1:00 p.m. it usually means you are not doing very well.’’

Mara went on to express his hope that the Giants give Grodnick a reason to attend the home games this season “regardless of what time they’re scheduled.’’

Mara closed by thanking Grodnick for the letter and reiterated his appreciation for Grodnick’s support of the Giants going back to when franchise patriarch Wellington Mara owned and ran the team. John Mara is the oldest child of the late Wellington Mara.

“Look, Wellington always answered every letter,” Grodnick said. “No matter if it was a 5-year-old kid telling him, ‘You stink,’ he would answer the letter. John follows the family tradition, I guess you could call it.

“I always tell him, ‘You’re the best owner in football.’ I’m not writing him nasty letters. I’m one of his customers. [NFL commissioner] Roger Goodell doesn’t care about us. We’re [Mara’s] customers, and we send our money to him.’’

Mara told The Post the mail he’s received from fans has been “very light’’ since the end of the season, which was a good one for the Giants. They broke a five-year cycle of lousy football and won a playoff game for the first time since after the 2011 season.

“For the first time in a few years, almost all of them have been very cordial,’’ Mara said.

The peak correspondence periods for Mara are during the season and especially leading into the NFL Draft.

“I get a lot of advice,’’ Mara acknowledged.

It comes as no surprise to anyone with a modicum of interaction on social media to learn that the worse the Giants are playing, the higher the volume of letters that come across Mara’s desk and into his email inbox.

“And yes, the letters can get nasty at times,’’ Mara said. “I do find that you can minimize the venom by just responding. Most people appreciate that.’’

This is a lesson Mara learned from his father, famous for penning handwritten notes to fans whose emotions ranged from ecstatic to irate and anywhere in between.

“I used to ask my father why he responded to the nasty letters,’’ Mara said. “He would constantly tell me, ‘They write because they care.’ ’’

The surefire evidence of that care?

“Almost every letter starts with ‘I’ve been a ticket holder or a fan since …,’” Mara said.

Last season, with the Giants going 9-7-1 to make the playoffs for the first time since 2016, Mara did not receive as many letters as in previous years. This says something about human nature, as far as assigning blame versus distributing praise.

Mara’s goal is to respond to every letter, but he admits “there are some that fall through the cracks.’’

Mara usually writes his response by hand and then has it typed out and mailed.  Sometimes, he sends the handwritten note.

Want to catch a game? The Giants schedule with links to buy tickets can be found here.

Grodnick, 75, recalls his father, Martin, purchased tickets from a family friend who owned a bunch of ducats in his name.

Martin, Les and his younger brother Jimmy (now 66 and living in Mobile, Ala.) would go to games as often as they could, including stops at the Yale Bowl in New Haven and at Shea Stadium until the Giants built their own stadium in East Rutherford.

When Les left New Jersey in 1966 to attend the University of Miami, Jimmy and his dad stayed behind and went to nearly every home game.

“We had tickets at the old Yankee Stadium where the bullpen was, and they were really good seats,’’ Grodnick said. “There was a wooden gondola in front of the seats, and Willie B. Williams from WNEW would crawl over there and sit in front of them.’’

Grodnick grew up in Maplewood, and retired from the family textile business in Milltown, N.J., in 1983. He makes a trip to training camp every summer, works his way onto the sideline and has schmoozed with Giants general managers dating back to the days of George Young. He once had lunch with Bill Parcells at Manny’s of Moonachie. The Giants have won four Super Bowls, and Grodnick has attended every one of them.

One year — Grodnick said he does not remember whether it was 2004 or 2006 — he approached Mara during a camp practice at the University at Albany. Mara was sitting with his kids on the bleachers.

“I asked him, we’ve had these seats from the ’50s, my dad died, this guy [whose name was on the tickets] moved away, I live in Florida now, is there any way we can get these seats split up and put in our names?’’ Grodnick said. “He knew exactly what to tell me. He said, ‘Look, whoever’s got the seats in his name, send me a certified letter, how you want the seats split up and I’ll take care of it.’ And he took care of it.’’

Grodnick already has this summer’s training camp trip planned, and figures he will attend five games in person this season at MetLife Stadium. As for the night games, well, Grodnick already has expressed to the head man what he thinks of traveling in from Miami for one of those evening affairs.

“Let John Mara invite me into his box,’’ Grodnick said. “Maybe I’ll go.’’

Here are two questions that have come up recently that we will attempt to answer as accurately as possible:

When will we see Saquon Barkley in uniform and on the field?

Not for a bit. Now that the Giants and Barkley cannot negotiate a long-term deal, there is no financial reason for Barkley to report to training camp. As long as he keeps the franchise tag tender unsigned, he is not under contract and cannot be fined for staying away.

Barkley had no leverage in these negotiations, and withholding his services is the only way for him to display his disapproval at having to play for the one-year tag of $10.1 million. In the short term, this should not have an adverse effect on Barkley or the Giants. He knows the offense, and he already is in impeccable shape. Saving wear and tear on his body is not a bad thing. He does need to get into “football shape,’’ which means putting on the pads and taking some hits.

Barkley was not going to play much, if at all, in the preseason games, so missing them is not a big deal. If he shows up in late August ready to roll, physically, mentally and emotionally, he should be fine for the Sept. 10 season opener against the Cowboys.

There is no way he will opt to miss games. That would cost him slightly more than $560,000 for every game he misses, and would not help him in his quest to increase his value for the 2024 season.

What will the first week of training camp look like?

The first practice is July 26. The Giants will work on the field from 10 a.m. until 11:45 or so for the first two days. There will be a later practice on July 28 (5 p.m. to 7 p.m.), giving the players’ bodies a little more time to rest and recover after the first two days.

The players get their first day off of camp on Saturday, July 29. Do not expect to see the team in pads the first three days. There is a ramp-up period in the early stages of camp.

Figure the first day in pads for the Giants will be Sunday, July 30, after they return from the off-day. That is the day all the linemen — especially the offensive linemen — look forward to, when the hitting begins.