🦉 UnitOwl

Days to Seconds

1 Day (d) = 86,400 Second (s)

Result
86,400 s
1 d = 86,400 s

How Many Seconds in a Day?

One day equals exactly 86,400 seconds (24 hours x 60 minutes x 60 seconds). To convert days to seconds, multiply the day value by 86,400. This conversion is ubiquitous in programming, server administration, scientific computing, and engineering. Database timestamps store time as seconds since an epoch (like Unix time, which counts seconds since January 1, 1970). Server administrators set SSL certificate lifetimes, cache durations, and log rotation periods in seconds. Scientists measure decay rates, growth periods, and reaction times that may span days but need to be expressed in seconds for equations. Knowing that 86,400 seconds per day is one of the most frequently used constants in software development and IT operations.

How to Convert Day to Second

  1. Start with your value in days.
  2. Multiply the day value by 86,400 to get seconds.
  3. For example, 7 days x 86,400 = 604,800 seconds.
  4. For partial days, multiply the fraction by 86,400: 0.5 days = 43,200 seconds (12 hours).
  5. Alternatively, convert days to hours (x 24), then to minutes (x 60), then to seconds (x 60). The combined factor is 24 x 60 x 60 = 86,400.

Real-World Examples

A cookie should expire in 30 days. What is the max-age value in seconds?
30 x 86,400 = 2,592,000 seconds.
A DNS TTL (time to live) should be 1 day. What value do you set?
1 x 86,400 = 86,400 seconds.
An SSL certificate expires in 90 days. How many seconds of validity does that represent?
90 x 86,400 = 7,776,000 seconds.
A satellite orbits Earth every 90 minutes. How many orbits does it complete in 1 day?
1 day = 86,400 seconds. 90 minutes = 5,400 seconds. 86,400 / 5,400 = 16 orbits per day.

Quick Reference

Day (d) Second (s)
1 86,400
2 172,800
5 432,000
10 864,000
25 2,160,000
50 4,320,000
100 8,640,000
500 43,200,000
1,000 86,400,000

History of Day and Second

The number 86,400 is the product of three ancient conventions: 24 hours per day (Egyptian), 60 minutes per hour (Babylonian), and 60 seconds per minute (also Babylonian). This constant became programmatically significant with the development of Unix in 1970, when Ken Thompson and Dennis Ritchie at Bell Labs chose to represent time as the number of seconds since midnight UTC on January 1, 1970 — the Unix epoch. This representation (Unix timestamp) uses 86,400 to convert between days and seconds and is now embedded in virtually every operating system, database, and programming language. The Unix timestamp will face a problem in 2038 when 32-bit systems overflow their signed integer limit, similar to the Y2K issue. Most modern systems use 64-bit timestamps, which will not overflow for approximately 292 billion years.

Common Mistakes to Avoid

  • Using 84,600 or 86,000 instead of 86,400. The exact value is important in programming — an off-by-one-thousand error in a cache TTL or cookie expiration can cause subtle and hard-to-debug issues.
  • Not accounting for leap seconds in high-precision applications. UTC occasionally adds a leap second, making a specific day 86,401 seconds long. Most applications can ignore this, but GPS systems, financial trading platforms, and astronomical software must handle it.

Frequently Asked Questions

What is Unix time and how does it relate to 86,400?
Unix time counts seconds since January 1, 1970 00:00:00 UTC. Each day adds 86,400 to the counter. For example, January 2, 1970 00:00:00 UTC = 86,400 in Unix time. As of early 2026, the Unix timestamp is approximately 1.77 billion seconds.
Are there really 86,400 seconds in every day?
For civil timekeeping, yes. But a solar day (measured by the Sun) varies slightly throughout the year due to Earth's elliptical orbit. Atomic clocks keep constant seconds, so occasionally a "leap second" is inserted, making that day 86,401 seconds. As of 2025, there have been 27 leap seconds since 1972.
What is the 2038 problem?
Systems using a signed 32-bit integer for Unix timestamps will overflow on January 19, 2038 at 03:14:07 UTC. At that moment, the timestamp exceeds 2,147,483,647 seconds (the maximum value for a 32-bit signed integer). This is analogous to Y2K. The fix is using 64-bit timestamps, which most modern systems already do.
Quick Tip

Memorize 86,400 as a constant — it appears so frequently in software development that it deserves a place in your mental toolkit alongside pi and the speed of light. Many codebases define it as a named constant (SECONDS_PER_DAY or DAY_IN_SECONDS) to improve readability.