Mental math

$365 days= 365 days \times 24 hours \times 60 minutes \times 60 seconds$ = $31536000 seconds$

Uh-huh, you expect me to do that in my head? Not me.

$365\; {\cancel{days}}\times\dfrac{24\; {\cancel{hours}}}{1\; {\cancel{day}}}\times\dfrac{60\; {\cancel{minutes}}}{1\; {\cancel{hour}}}\times\dfrac{60\; seconds}{1\; {\cancel{minute}}}=365\times 24\times 60^2=\boxed{31536000}$

their is 86400 seconds in s day, times that with 365, it's 31536000. 31536000 seconds in a 365 day year.

One year is not only 365 days. Where is additional 6 hour remnant?

As we know:
● 1 minute $=60$ seconds
● 60 minutes or 1 hour $=60×60$ seconds
● 24 hours or 1 day $=60×60×24$ seconds
● Now, 365 days or 1 year $=60×60×24×365$ seconds $=\boxed{31536000}$