Answer:
This is not as straightforward as it initially looks. It surely does look simple at first glance: Just multiply the number of days per year with the number of hours per day with the number of minutes per hour with the number of seconds per minute, right? Nope, it is not that easy. Neither the number of days per year nor the number of hours a day nor number of seconds per minute is constant:
Days per year:
In the Gregorian calendar, a year either has 365 calendar days (common years) or 366 calendar days (leap years). As there are 97 leap years in 400 years, the average is 365.2425 days per year. It still is a bit of a stretch to calculate an average as the Gregorian calendar has only been in use for barely more than the period length of 400 years.
The average does not help you, however, if you want to calculate the number of seconds in a number of years that’s not divisible by 400, such as the seven years 2015 to 2021—that’s exactly 2,557 days, not 7 × 365.2425 days.
There were also some special years where the number of days was different. When the Gregorian calendar was introduced, countries switched from the Julian calendar at different times by skipping 10 to 13 days, resulting in years as short as 352 days (1918 in Russia). In one case, the attempts to convert to the Gregorian calendar even resulted in a double leap year with 367 days (1712 in Sweden).
In addition, there are some territories near the International Date Line that switched sides by skipping a day (e.g., 30 December 2011 in Samoa, resulting in a 364 day year) or repeating a day (4 July 1892 in Samoa, resulting in a 367 day leap year).
Hours per day:
The number of hours per calendar day is not always 24. Where summer time (daylight saving time) is used, there is a day with 23 hours in spring and one with 25 hours in autumn (or 23.5 and 24.5 hours in Lord Howe Island, which only sets its clocks forward by half an hour in summer—and yes, that’s summer on the southern hemisphere).
That does not really matter if you talk about years starting on 1 January, 00:00 hrs. and ending on 31 December, 24:00 hrs. It does matter, however, if you are looking at a one-year period that starts in summer time and ends in standard time—and this can actually happen where summer time does not always starts on the same date, e.g. in Europe 28 March 2015 (one day before the switch to summer time) to 28 March 2016 (one day after the switch to summer time).
Then, it happens occasionally that a country or region switches time zones. This may also result in some extra or missing hours (or portions of a hour) in the respective year.
Minutes per hour:
Surprisingly, this always seems to be 60. Yeah!
Seconds per minute:
This is normally 60. Occasionally, however, there are leap seconds, resulting in a calendar minute of 61 seconds or 59 seconds (the later has not happened… yet).
Conclusion:
You cannot simply convert years into seconds. You have to define the start date and time, add the number of years to the year part of that timestamp, beware of gaps in the calendar (29 February in common years, or the missing hour during the switch to summer time), convert both start and end date separately to UTC, and then start counting leap years and leap seconds.
