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.