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 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.