2.8 日期

注意观察时间转化

Sys.Date()
## [1] "2023-05-25"
Sys.time()
## [1] "2023-05-25 04:51:07 UTC"
c(Sys.time(), Sys.Date())
## [1] "2023-05-25 04:51:07 UTC" "2023-05-25 00:00:00 UTC"
data.table::year(Sys.Date())
## [1] 2023
data.table::year(Sys.time())
## [1] 2023
data.table::year(c(Sys.time(), Sys.Date()))
## [1] 2023 2023
x <- Sys.time()
class(x)
## [1] "POSIXct" "POSIXt"
format(x, format = "%Y-%m-%d")
## [1] "2023-05-25"
x <- c("2019-12-21", "2019/12/21")
data.table::year("2019-12-21")
## [1] 2019
data.table::year("2019/12/21")
## [1] 2019

但是，下面这样会报错

data.table::year(x)
## Error in as.POSIXlt.character(x): character string is not in a standard unambiguous format

正确的姿势是首先将表示日期的字符串格式统一

gsub(pattern = "/", replacement = "-", x) |>  
  data.table::year()
## [1] 2019 2019

date-times 表示 POSIXct 和 POSIXlt 类型的日期对象

(x <- Sys.time())
## [1] "2023-05-25 04:51:07 UTC"
class(x)
## [1] "POSIXct" "POSIXt"
data.table::second(x) # 取秒
## [1] 7
format(x, format = "%S")
## [1] "07"
data.table::minute(x) # 取分
## [1] 51
format(x, format = "%M")
## [1] "51"
data.table::hour(x) # 取时
## [1] 4
format(x, format = "%H")
## [1] "04"
data.table::yday(x) # 此刻在一年的第几天
## [1] 145
data.table::wday(x) # 此刻在一周的第几天，星期日是第1天，星期六是第7天
## [1] 5
data.table::mday(x) # 此刻在当月第几天
## [1] 25
format(x, format = "%d")
## [1] "25"
weekdays(x)
## [1] "Thursday"
weekdays(x, abbreviate = T)
## [1] "Thu"
data.table::week(x) # 此刻在第几周
## [1] 21
data.table::isoweek(x)
## [1] 21
data.table::month(x) # 此刻在第几月
## [1] 5
format(x, format = "%m")
## [1] "05"
months(x)
## [1] "May"
months(x, abbreviate = T)
## [1] "May"
data.table::quarter(x) # 此刻在第几季度
## [1] 2
quarters(x)
## [1] "Q2"
data.table::year(x) # 取年
## [1] 2023
format(x, format = "%Y")
## [1] "2023"

format() 是一个泛型函数，此刻命名空间有 101 方法。 format.Date()， format.difftime()， format.POSIXct() 和 format.POSIXlt() 四个函数通过格式化不同类型的日期数据对象抽取指定部分。

format(difftime(Sys.time(), x, units = "secs"))
## [1] "0.08600068 secs"

日期转化详见 [9], [10]

上个季度最后一天

# https://d.cosx.org/d/421162/16
as.Date(cut(as.Date(c("2020-02-01", "2020-05-02")), "quarter")) - 1
## [1] "2019-12-31" "2020-03-31"

本季度第一天

as.Date(cut(as.Date(c("2020-02-01", "2020-05-02")), "quarter"))
## [1] "2020-01-01" "2020-04-01"

类似地，本月第一天和上月最后一天

# 本月第一天
as.Date(cut(as.Date(c("2020-02-01", "2020-05-02")), "month"))
## [1] "2020-02-01" "2020-05-01"
# 上月最后一天
as.Date(cut(as.Date(c("2020-02-01", "2020-05-02")), "month")) - 1
## [1] "2020-01-31" "2020-04-30"

timeDate 提供了很多日期计算函数，比如季初、季末、月初、月末等

library(timeDate) 
# 季初
as.Date(format(timeFirstDayInQuarter(charvec = c("2020-02-01", "2020-05-02")), format = "%Y-%m-%d")) 
# 季末
as.Date(format(timeLastDayInQuarter(charvec = c("2020-02-01", "2020-05-02")), format = "%Y-%m-%d"))
# 月初
as.Date(format(timeFirstDayInMonth(charvec = c("2020-02-01", "2020-05-02")), format = "%Y-%m-%d")) 
# 月末
as.Date(format(timeLastDayInMonth(charvec = c("2020-02-01", "2020-05-02")), format = "%Y-%m-%d"))

cut.Date() 是一个泛型函数，查看它的所有 S3 方法

methods(cut)
## [1] cut.Date        cut.default     cut.dendrogram* cut.IDate*     
## [5] cut.POSIXt     
## see '?methods' for accessing help and source code

格式化输出日期类型数据

formatC(round(runif(1, 1e8, 1e9)), digits = 10, big.mark = ",")
## [1] "586,715,784"
# Sys.setlocale(locale = "C") # 如果是 Windows 系统，必须先设置，否则转化结果是 NA
as.Date(paste("1990-January", 1, sep = "-"), format = "%Y-%B-%d")
## [1] "1990-01-01"

获取当日零点

format(as.POSIXlt(Sys.Date()), "%Y-%m-%d %H:%M:%S")
## [1] "2023-05-25 00:00:00"

从 POSIXt 数据对象中，抽取小时和分钟部分，返回字符串

strftime(x = Sys.time(), format = "%H:%M")
## [1] "04:51"
表 2.3: 日期表格
代码 含义 代码 含义
%a Abbreviated weekday %A Full weekday
%b Abbreviated month %B Full month
%c Locale-specific date and time %d Decimal date
%H Decimal hours (24 hour) %I Decimal hours (12 hour)
%j Decimal day of the year %m Decimal month
%M Decimal minute %p Locale-specific AM/PM
%S Decimal second %U Decimal week of the year (starting on Sunday)
%w Decimal Weekday (0=Sunday) %W Decimal week of the year (starting on Monday)
%x Locale-specific Date %X Locale-specific Time
%y 2-digit year %Y 4-digit year
%z Offset from GMT %Z Time zone (character)

本节介绍了 R 本身提供的基础日期操作，第??章着重介绍一般的时间序列类型的数据对象及其操作。

参考文献

[9]
B. D. Ripley and K. Hornik, “Date-time classes,” R News, vol. 1, no. 2, pp. 8–11, 2001,Available: https://cran.r-project.org/doc/Rnews/Rnews_2001-2.pdf
[10]
G. Grothendieck and T. Petzoldt, “R Help Desk: Date and time classes in R,” R News, vol. 4, no. 1, pp. 29–32, 2004,Available: https://www.r-project.org/doc/Rnews/Rnews_2004-1.pdf