Description of time span classes in lubridate.

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Description

A time span can be measured in three ways: as a duration, an interval, or a period.

Details

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 dweeks, ddays, dhours, dminutes and 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 new date-time.

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 years, months, weeks, days, minutes, 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.

See Also

duration for creating duration objects and period for creating period objects, and interval for creating interval objects.

Examples

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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)

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