The year is a unit of time that is a multiple of an SI unit and uses the symbol a.
The Julian year is made up of exactly 365.25 days – each with 60 x 60 x 24 seconds (86,400 seconds). The .25 days is worked into the system by counting 366 days once every 4 cycles. This is known as a “leap year” and the “leap day” happens at the end of February.
The term year represents the length of time it takes for the earth to complete one full cycle around the sun. Each planet therefore has a different year length.
To track years, humanity has assigned an incremental numbering system. Depending on which culture, religion or part of the world you are from or follow, this number varies. The most common numbering system suggests we are in the 21st century – i.e. in the 2000’s. This system started 0 AD (Anno Domini – which translates from Latin as “In the year of our Lord”). Time before this is referred to as BC (before Christ) and the number increases as you go further into history (like a negative number would).
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.