A time span can be measured in three ways: as a duration, an interval, or a period.
Durations record the exact number of seconds in a time span. They measure the exact passage of time but do not always align with measurements made in larger units of time such as hours, months and years. This is because the exact length of larger time units can be affected by conventions such as leap years and Daylight Savings Time. Base R measures durations with the difftime class. lubridate provides an additional class, the duration class, to facilitate working with durations.
durations display as the number of seconds that occur during a time span. If the number is large, a duration object will also display the length in a more convenient unit, but these measurements are only estimates given for convenience. The underlying object is always recorded as a fixed number of seconds. For display and creation purposes, units are converted to seconds using their most common lengths in seconds. Minutes = 60 seconds, hours = 3600 seconds, days = 86400 seconds. Units larger than days are not used due to their variability.
duration objects can be easily created with the helper functions
dseconds. These objects can be added to and subtracted from date-
times to create a user interface similar to object oriented programming.
Duration objects can be added to Date, POSIXct, and POSIXlt objects to return a
Periods record the change in the clock time between two date-times. They are
measured in common time related units: years, months, days, hours, minutes,
and seconds. Each unit except for seconds must be expressed in integer
values. With the exception of seconds, none of these units have a fixed
length. Leap years, leap seconds, and Daylight Savings Time can expand or
contract a unit of time depending on when it occurs. For this reason, periods
do not have a fixed length until they are paired with a start date. Periods
can be used to track changes in clock time. Because periods
have a variable length, they must be paired with a start date
as an interval (
as.interval) before they can be
accurately converted to and from durations.
Period objects can be easily created with the helper functions
seconds. These objects
can be added to and subtracted to date-times to create a user interface
similar to object oriented programming. Period objects can be added to Date,
POSIXct, and POSIXlt objects to return a new date-time.
Intervals are time spans bound by two real date-times. Intervals can be
accurately converted to periods and durations. Since an interval is anchored
to a fixed moment of time, the exact length of all units of
time during the interval can be calculated. To
accurately convert between periods and durations, a period or duration should
first be converted to an interval with
as.interval. An interval displays as the start
and end points of the time span it represents.
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duration(3690, "seconds") period(3690, "seconds") period(second = 30, minute = 1, hour = 1) interval(ymd_hms("2009-08-09 13:01:30"), ymd_hms("2009-08-09 12:00:00")) date <- as.POSIXct("2009-03-08 01:59:59") # DST boundary date + days(1) date + ddays(1) date2 <- as.POSIXct("2000-02-29 12:00:00") date2 + years(1) # self corrects to next real day date3 <- as.POSIXct("2009-01-31 01:00:00") date3 + c(0:11) * months(1) span <- date2 %--% date #creates interval date <- as.POSIXct("2009-01-01 00:00:00") date + years(1) date - days(3) + hours(6) date + 3 * seconds(10) months(6) + days(1)