如何计算Growstart与Growend间的月份数（起始月>结束月场景）

我们需要计算growstart（生长季起始月）与growend（生长季结束月）之间的月份数，但当起始月大于结束月（如12月到次年4月）时，直接转换为日期计算会得到负数结果。以下是相关数据集及尝试过的代码：

数据集 #

structure(list(gid = c(100468L, 100468L, 100468L, 100468L, 100468L, 
104078L, 104078L, 104078L, 104078L, 104078L, 104078L, 115615L, 
115615L, 115615L, 115615L, 115615L, 115615L), country = c("Namibia", 
"Namibia", "Namibia", "Namibia", "Namibia", "Namibia", "Namibia", 
"Namibia", "Namibia", "Namibia", "Namibia", "Congo, Democratic Republic of (Zaire)", 
"Congo, Democratic Republic of (Zaire)", "Congo, Democratic Republic of (Zaire)", 
"Congo, Democratic Republic of (Zaire)", "Congo, Democratic Republic of (Zaire)", 
"Congo, Democratic Republic of (Zaire)"), xcoord = c("13.75", 
"13.75", "13.75", "13.75", "13.75", "18.75", "18.75", "18.75", 
"18.75", "18.75", "18.75", "27.25", "27.25", "27.25", "27.25", 
"27.25", "27.25"), ycoord = c("-20.25", "-20.25", "-20.25", "-20.25", 
"-20.25", "-17.75", "-17.75", "-17.75", "-17.75", "-17.75", "-17.75", 
"-9.75", "-9.75", "-9.75", "-9.75", "-9.75", "-9.75"), km2 = c("3282.64642089", 
"3282.64642089", "3282.64642089", "3282.64642089", "3282.64642089", 
"3233.1051487", "3233.1051487", "3233.1051487", "3233.1051487", 
"3233.1051487", "3233.1051487", "3122.97751422", "3122.97751422", 
"3122.97751422", "3122.97751422", "3122.97751422", "3122.97751422"
), maincrop = c(52L, 52L, 52L, 52L, 52L, 37L, 37L, 37L, 37L, 
37L, 37L, 37L, 37L, 37L, 37L, 37L, 37L), crop = c("Others annual", 
"Others annual", "Others annual", "Others annual", "Others annual", 
"Cassava", "Cassava", "Cassava", "Cassava", "Cassava", "Cassava", 
"Cassava", "Cassava", "Cassava", "Cassava", "Cassava", "Cassava"
), growstart = c(12L, 12L, 12L, 12L, 12L, 9L, 9L, 9L, 9L, 9L, 
9L, 5L, 5L, 5L, 5L, 5L, 5L), growend = c(4L, 4L, 4L, 4L, 4L, 
4L, 4L, 4L, 4L, 4L, 4L, 12L, 12L, 12L, 12L, 12L, 12L), Time = structure(c(4352, 
4383, 4717, 4414, 4442, 9862, 10135, 10166, 10196, 9893, 9921, 
13057, 13088, 13118, 12904, 12935, 12965), class = "Date"), harvestYear = c(1982L, 
1982L, 1983L, 1982L, 1982L, 1997L, 1998L, 1998L, 1998L, 1997L, 
1997L, 2005L, 2005L, 2005L, 2005L, 2005L, 2005L)), row.names = c(1L, 
2L, 3L, 4L, 5L, 40000L, 40001L, 40002L, 40003L, 40004L, 40005L, 
200000L, 200001L, 200002L, 200003L, 200004L, 200005L), class = "data.frame")

尝试过的代码 #

vhi.crop.gs$Datestart<-gsub(" ","",paste(vhi.crop.gs$harvestYear,"-",vhi.crop.gs$growstart,"-","01"))
vhi.crop.gs$Datestart<-as.Date(vhi.crop.gs$Datestart,format="%Y-%m-%d")

vhi.crop.gs$Dateend<-gsub(" ","",paste(vhi.crop.gs$harvestYear,"-",vhi.crop.gs$growend,"-","01"))
vhi.crop.gs$Dateend<-as.Date(vhi.crop.gs$Dateend,format="%Y-%m-%d")

vhi.crop.gs %>%
  group_by(gid, country, xcoord, ycoord, crop, maincrop, harvestYear) %>%
  mutate(Months_difference = (interval(ymd(Datestart), 
                                     ymd(Dateend))) %/% months(1))

解决方案 #

方法一：直接通过月份数值计算 #

无需转换为日期，直接根据月份大小判断是否跨年度，计算月份差：

library(dplyr)

vhi.crop.gs <- vhi.crop.gs %>%
  mutate(
    Months_difference = ifelse(growstart > growend,
                               (12 - growstart) + growend,
                               growend - growstart)
  )

方法二：修正日期年份后计算 #

如果需要保留日期格式用于后续操作，可根据生长季是否跨年度调整日期的年份：

library(lubridate)
library(dplyr)

vhi.crop.gs <- vhi.crop.gs %>%
  mutate(
    # 跨年度时，起始年份为收获年份减1，否则为收获年份
    Datestart = ymd(paste(ifelse(growstart > growend, harvestYear - 1, harvestYear), growstart, "01", sep = "-")),
    # 跨年度时，结束年份为收获年份，否则为收获年份
    Dateend = ymd(paste(ifelse(growstart > growend, harvestYear, harvestYear), growend, "01", sep = "-")),
    Months_difference = interval(Datestart, Dateend) %/% months(1)
  )

两种方法都能解决跨年度生长季月份差为负的问题：方法一简洁高效，适合仅需月份差的场景；方法二保留了正确的日期序列，适合需要后续日期处理的场景。

内容的提问来源于stack exchange，提问作者Shunrei