Users must specify which time units to measure the period
in. The exact length of each time unit in a period will
depend on when it occurs. See Period-class
and new_period
. The choice of units is not
trivial; units that are normally equal may differ in
length depending on when the time period occurs. For
example, when a leap second occurs one minute is longer
than 60 seconds. Because periods do not have a fixed length, they can not
be accurately converted to and from Duration objects.
Duration objects measure time spans in exact numbers of
seconds, see Duration-class
. Hence, a one
to one mapping does not exist between durations and
periods. When used with a Duration object, as.period
provides an inexact estimate; the duration is broken into
time units based on the most common lengths of time
units, in seconds. Because the length of months are
particularly variable, a period with a months unit can
not be coerced from a duration object. For an exact
transformation, first transform the duration to an
interval with as.interval
.