I stand by my previous statement: you should avoid thinking of difftime objects
as having units. At the point when you learn to regard the units argument as
being associated with the numeric values you pass TO as.difftime or that you
extract FROM a difftime object with as.numeric, then you will understand
POSIXct arithmetic.
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Sent from my phone. Please excuse my brevity.
"MacQueen, Don" <macque...@llnl.gov> wrote:
>The difftime() function has a units argument, so it seems kind of
>obvious
>to me to think of objects of class 'difftime' as having units. And
>indeed
>they do, stored as an attribute:
>
>> str(difftime( Sys.time()+3700, Sys.time(), units='min'))
>Class 'difftime' atomic [1:1] 61.7
> ..- attr(*, "units")= chr "mins"
>
>> str(difftime( Sys.time()+3700, Sys.time(), units='sec'))
>Class 'difftime' atomic [1:1] 3700
> ..- attr(*, "units")= chr "secs"
>
>The numeric values are already in the specified units, prior to any
>printing or other further use of the result.
>
>
>?difftime says,
>
>-quote-
>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.
>
>
>-end quote-
>
>So it's pretty clear that the units are chosen when the difference is
>calculated, not later. Inspection of the difftime() function source
>code
>confirms this. One can also inspect print.difftime() and see that the
>numeric value is used as is, without any unit conversion.
>
>
>Which returns me to my original point: when using diff() on a POSIXt
>object, I don't know how to specify the units, whereas with difftime()
>I
>do. If there is a way to specify units using diff(), I'd love to know
>about it!
>
>
>-Don
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