That's how many seconds are in a day.
Here's the math: 60 seconds/minute X 60 minutes/hour X 24 hours per day.
That piece of trivia can be a tech-y or a science-y thing.
And some wax philosophical about it--as that is the time allotted for any of us per day.
(Reference: https://youtu.be/4SHFp3qWDZc )
But the timestamp from 0 to 86,399 seconds is critical for many technical operations. Such is the case for operations on a Mission Control Team that tracks satellites.
In S*T*C, one of main characters, Sam Clark, with her co-workers John McHenry, Tan Ho, and Enrique Rodriguez, get a tour of the Mission Control Complex on their first day on the job.
Their lead Paul Coleman, also known as PC, introduced to them to the timestamp:
Sam pointed to the large red neon display reading 1984.040:76879 with the lowest digits incrementing.
“And what do those numbers mean?”
“It’s a fancy clock.” PC explained, “Nineteen eighty-four, the year. Forty, the Julian day. That’s February ninth, since it’s been forty days since the New Year. After the colon there, 76891 is the time in seconds.”
PC looked at his watch. “Which is…approximately 2122 in military time. For us civilians that’s 9:22 at night.”
Sam looked puzzled. “But it’s going on 1:30 in the afternoon.”
“Ah!” Paul raised a finger. “But it’s almost 9:30 p.m. in England. And all our times are recorded in Zulu time.”
“Zulu?” Enrique piped up. “Man. I loved that movie. A classic with Michael Caine—”
PC grinned. “Zulu is the phonetic alphabet term for the Zth time zone. It’s the same as Greenwich Mean Time. GMT. Or Universal Time. UT. All the scientists as well as the military use it. Close to shift change…that’s four o’clock in the afternoon here…Pacific Standard Time…the clocks all roll over to quad zero. Or 0000z.”
PC pointed to the red clock. “And that clock will roll over from 040:86399 to 041:00000. A new day.”
“Why 86—” Tan had forgotten the rest of the digits in the five-digit number.
John chimed in, “Because there are 86,400 seconds in a day.”
“But, ah! Except when we have to adjust for the leap second.”
Then PC looked to the senior PA ensconced at an F2 console. “Ned?” he asked. “Will you explain what y’all are doing here?”
“Good question, PC. What am I doing here?”
Then Ned Gaubert gave a quiet sigh and turned around with his mouth askew. “This is called a pass. And it’s not what we make at girls.”
“Good thing.” Sam returned a sidelong smile. “I’m married.”
Smith, S K. S*T*C (pp. 35-36). Kindle Edition.