Function `difftime`

calculates a difference of two date/time
objects and returns an object of class `"difftime"`

with an
attribute indicating the units. The
`Math`

group method provides
`round`

, `signif`

, `floor`

,
`ceiling`

, `trunc`

, `abs`

, and
`sign`

methods for objects of this class, and there are
methods for the group-generic (see
`Ops`

) logical and arithmetic
operations.

If `units = "auto"`

, a suitable set of units is chosen, the largest
possible (excluding `"weeks"`

) in which all the absolute
differences are greater than one.

Subtraction of date-time objects gives an object of this class,
by calling `difftime`

with `units = "auto"`

. Alternatively,
`as.difftime()`

works on character-coded or numeric time
intervals; in the latter case, units must be specified, and
`format`

has no effect.

Limited arithmetic is available on `"difftime"`

objects: they can
be added or subtracted, and multiplied or divided by a numeric vector.
In addition, adding or subtracting a numeric vector by a
`"difftime"`

object implicitly converts the numeric vector to a
`"difftime"`

object with the same units as the `"difftime"`

object. There are methods for `mean`

and
`sum`

(via the `Summary`

group generic), and `diff`

via `diff.default`

building on the `"difftime"`

method for arithmetic, notably
`-`

.

The units of a `"difftime"`

object can be extracted by the
`units`

function, which also has a replacement form. If the
units are changed, the numerical value is scaled accordingly. The
replacement version keeps attributes such as names and dimensions.

Note that `units = "days"`

means a period of 24 hours, hence
takes no account of Daylight Savings Time. Differences in objects
of class `"Date"`

are computed as if in the UTC time zone.

The `as.double`

method returns the numeric value expressed in
the specified units. Using `units = "auto"`

means the units of the
object.

The `format`

method simply formats the numeric value and appends
the units as a text string.