Time is a funny thing. We treat a day like this solid, unchangeable block of existence, but the moment you start talking to a software engineer or a physicist, that certainty starts to crumble. Honestly, if you're just here for the quick math because you're writing a script or setting a TTL (Time to Live) value for a database cache, the number you need for 24 hours in milliseconds is 86,400,000.
There. That's the raw data.
But if you actually plug that hardcoded number into a high-stakes global system without thinking about how time actually works, you might just break something. It sounds dramatic, but time isn't just math; it’s geography, politics, and planetary wobble all mashed into one.
The basic math of 24 hours in milliseconds
Let's break down how we get there. It’s a simple multiplication chain, but it’s worth seeing the logic so you don't have to Google it next time. Most of us know there are 60 seconds in a minute. From there, we have 60 minutes in an hour. Multiply those ($60 \times 60$) and you get 3,600 seconds in a single hour.
Now, since we are looking for a full day, we take those 3,600 seconds and multiply them by 24. That gives us 86,400 seconds.
Computers don't really "think" in seconds for most high-precision tasks, though. They use milliseconds (ms), which are thousandths of a second. So, you take your 86,400 seconds, tack on three zeros, and you arrive at the magic number: 86,400,000 ms.
It looks clean. It looks perfect. It is, unfortunately, a bit of a lie.
When 86,400,000 isn't actually a day
If you're building a simple countdown timer for a workout app, 86,400,000 milliseconds is fine. Use it. Forget I said anything. But if you are working in DevOps or backend engineering, you’ve probably heard of the "Leap Second."
The Earth is a bit of a chaotic rotator. It’s not a perfect clock. Because of tidal friction and changes in the Earth's core, our planet occasionally slows down or speeds up. To keep our atomic clocks (UTC) in sync with the actual rotation of the Earth (UT1), the International Earth Rotation and Reference Systems Service (IERS) sometimes adds an extra second to a day.
📖 Related: The Truth About Photos of the Deepest Part of the Ocean
When that happens, a day isn't 86,400,000 milliseconds. It’s 86,401,000 milliseconds.
You might think, "So what? It's one second." Tell that to Reddit, Cloudflare, or Qantas Airways. All of them have suffered massive technical outages because their systems couldn't handle a minute with 61 seconds in it. When a server expects the clock to always move forward in a predictable 86.4-million-millisecond loop and suddenly it doesn't, things tend to catch fire.
The Daylight Savings Trap
Then there's the human element: Daylight Savings Time (DST). This is the absolute bane of existence for anyone handling 24 hours in milliseconds in a scheduling app.
In the United States and many other regions, we "spring forward" and "fall back." On the day we spring forward, a "day" is technically only 23 hours long. That’s 82,800,000 milliseconds. When we fall back in autumn, the day is 25 hours long, or 90,000,000 milliseconds.
If you hardcode 86,400,000 into a system that sends out daily medication reminders or automated bank transfers, your timing will be off by exactly one hour twice a year. People miss pills. Transactions fail. It’s a mess.
How the pros handle time durations
Most senior developers will tell you: Never roll your own time logic. Instead of manually calculating 86.4 million, they use libraries that understand the edge cases. In JavaScript, you might use Luxon or date-fns. In Python, you'd lean on timedelta. These tools know about leap years, leap seconds, and the weirdness of Time Zones.
For example, in many modern languages, instead of writing 86400000, you’d write something like Duration.ofDays(1).toMillis(). It’s more readable, and more importantly, it signals to the next person reading your code that you're treating "a day" as a conceptual unit of time rather than a fixed mathematical constant.
The precision problem
Why do we even use milliseconds? Why not microseconds or nanoseconds?
Mostly, it’s a middle ground. Milliseconds provide enough precision for most human-centric activities (like clicking a button or loading a video) without the massive storage overhead of nanosecond precision. Most "Epoch Time" timestamps—the number of units passed since January 1, 1970—are stored in milliseconds.
If you look at a Unix timestamp right now, it’s a massive number. Adding 86,400,000 to that number is the standard way to calculate "tomorrow at this exact time."
💡 You might also like: Everything You Need to Know About Schedule 80 PVC Dimensions (and Why They Matter)
Real-world applications of the 86.4 million constant
You'll see this number pop up in places you wouldn't expect.
- Video Games: Many game engines use millisecond "ticks" to handle daily rewards or stamina refills. If the game feels laggy, it might be because the server is struggling to process all those millisecond-level updates across thousands of players.
- Financial Markets: High-frequency trading happens in microseconds, but daily settlement cycles often rely on the standard 24-hour millisecond count to trigger end-of-day reports.
- Web Caching: If you want a website to "remember" a piece of data for exactly one day, you set the
max-ageor expiration header. Many content delivery networks (CDNs) ask for this value in milliseconds or seconds.
Actionable steps for working with 24-hour durations
If you are actually implementing this in a project, don't just copy-paste the number and walk away. Follow these rules to make sure your system doesn't break when the world changes.
- Store in UTC: Never, ever store your "day" calculations in local time. Always convert to Coordinated Universal Time (UTC) first. This avoids the Daylight Savings nightmare entirely.
- Use Constants: If you must use the raw number, define it at the top of your file as a constant:
const MS_PER_DAY = 86400000;. This makes it clear what the number represents. - Account for Drift: If you are building a physical device (like an IoT sensor), remember that hardware clocks "drift." They aren't perfect. Over a year, those 86,400,000 milliseconds per day might start to lag behind the real world by several seconds. Use Network Time Protocol (NTP) to sync back up.
- Think about "Wall Time" vs. "Absolute Time": Ask yourself if you want your event to happen after 86.4 million milliseconds have passed (absolute), or if you want it to happen when the clock hits 12:00 PM again (wall time). They are not always the same thing.
Understanding 24 hours in milliseconds is the first step, but respecting the complexity of time is what separates a hobbyist from a pro. Use the number 86,400,000 when you need a quick fix, but use a dedicated time library when you're building something that actually needs to last.