The day is a unit of time and is an SI-derived unit with the symbol d.
On Earth, it is defined as 86,400 seconds and is approximately the time it takes for the earth to complete a full rotation around its axis. In the earlier days, this was measured by waiting for a cast shadow to match a template drawn from the previous day's shadow.
There are 365 days in a year and, on average, 30.42 days in a month.
The unit day has many different variants; depending on what is used to measure the Earth's rotation. In a sidereal day (a rotation with respect to a distant star or constellation, not the sun), there is actually 4 minutes less than 24 hours in a cycle.
The second is the SI base unit for time and has the symbol s.
The second is commonly understood to be 1/86400 of a day; there are 60 seconds in a minute, 60 minutes in an hour and 24 hours in a day.
Analog and digital watches and clocks almost all measure and display the progression of time via the use of a second counter / hand - and is generally considered the lowest denomination of time.
The earliest display and use of seconds was in the last half of the 16th century. Prior to this, it was not considered accurate enough to measure in seconds as a mechanical device was needed to ensure consistency.
In 1656, a Dutch scientist invented the first pendulum clock that measured seconds. His name was Christiaan Huygens.