How to Get the Same Weekday Last Year Given any Given Year?

问题

I would like to get the same day last year given any year. How can I best do this in R. For example, given Sunday 2010/01/03, I would like to obtain the Sunday of the same week the year before.

# "Sunday"
weekdays(as.Date("2010/01/03", format="%Y/%m/%d"))

# "Saturday"
weekdays(as.Date("2009/01/03", format="%Y/%m/%d"))

回答1:

To find the same weekday one year ago, simply subtract 52 weeks or 364 days from the given date:

d <- as.Date("2010-01-03")
weekdays(d)
#[1] "Sunday"
d - 52L * 7L
#[1] "2009-01-04"
weekdays(d - 52L * 7L)
#[1] "Sunday"

Please note that the calendar year has 365 days (or 366 days in a leap year) which is one or two days more than 52 weeks. So, the calendar date of the same weekday one year ago moves on by one or two days. (Or, it explains why New Year's Eve is always on a different weekday.)

回答2:

Using lubridate the following formula will give you the corresponding weekday in the same week in the previous year:

as.Date(dDate - 364 - ifelse(weekdays( dDate - 363) == weekdays( dDate ), 1, 0))

Where dDate is some date, i.e. dDate <- as.Date("2016-02-29"). The ifelse accounts for leap years.

回答3:

Look at ?years in package lubridate. This creates a period object which correctly spans a period, across leap years.

> library(lubridate)
> # set the reference date
> d1 = as.Date("2017/01/03", format="%Y/%m/%d")
> 
> # verify across years and leap years
> d1 - years(1)
[1] "2016-01-03"
> d1 - years(2)
[1] "2015-01-03"
> d1 - years(3)
[1] "2014-01-03"
> d1 - years(4)
[1] "2013-01-03"
> d1 - years(5)
[1] "2012-01-03"
> 
> weekdays(d1 - years(1))
[1] "Sunday"
> weekdays(d1 - years(2))
[1] "Saturday"
> 
> # feb 29 on year period in yields NA
> ymd("2016/02/29") - years(1)
[1] NA
> 
> # feb 29 in a non-leap year fails to convert
> ymd("2015/02/29") - years(1)
[1] NA
Warning message:
All formats failed to parse. No formats found. 
> 
> # feb 29, leap year with 4 year period works.
> ymd("2016/02/29") - years(4)
[1] "2012-02-29"
>

来源：https://stackoverflow.com/questions/41775077/how-to-get-the-same-weekday-last-year-given-any-given-year

标签

date

dayofweek