period_to_seconds
approximately converts a period to seconds assuming
there are 364.25 days in a calendar year and 365.25/12 days in a month.
seconds_to_period
create a period that has the maximum number of
non-zero elements (days, hours, minutes, seconds). This computation is exact
because it doesn't involve years or months.
period_to_seconds(x)
seconds_to_period(x)