Interval is an S4 class that extends the Timespan class. An Interval object records one or more spans of time. Intervals record these timespans as a sequence of seconds that begin at a specified date. Since intervals are anchored to a precise moment of time, they can accurately be converted to Period or Duration class objects. This is because we can observe the length in seconds of each period that begins on a specific date. Contrast this to a generalized period, which may not have a consistent length in seconds (e.g. the number of seconds in a year will change if it is a leap year).
Intervals can be both negative and positive. Negative intervals progress backwards from the start date; positive intervals progress forwards.
Interval class objects have two slots: .Data, a numeric object equal to the number of seconds in the interval; and start, a POSIXct object that specifies the time when the interval starts.