We multiply the number of seconds in one day with the total number of days.
The number of seconds in one year will depend on the type of calendar you use when approximating time. A Gregorian calendar year consists of 365 days and 1 day has 24 hours, each hour consists of 60 minutes and each minute has 60 seconds. To determine the number of seconds that are in one year, you have to know how many seconds are in one day, one week or one month before they can be summed up to get the overall seconds that make up a year.
The number of seconds in one hour is 3,600s (60 x 60). We multiply this value by 24 hours to get the total number of seconds in one day (24 x 3600) = 86400. Afterwards, you multiply by the total number of days depending on the type of the Calendar year.