Convert days to seconds

The day equals 86,400 seconds — the mean time for Earth to complete one rotation relative to the Sun (solar day). A sidereal day (relative to distant stars) is about 4 minutes shorter: 23 h 56 min 4 s. Most practical usage means solar day.

The second is the SI base unit of time. It is defined by the caesium-133 atomic transition: 9,192,631,770 cycles of this oscillation equal one second. Modern atomic clocks maintain the second to about 10⁻¹⁶ accuracy — you would lose one second in 100 million years.