Converting Date in Year.Decimal Form in R

Converting date in Year.decimal form in R

The lubridate package has a function, date_decimal that you can use for this.

x <- c(1988.0, 1988.25, 1988.5, 1988.75)
library(lubridate)
(f <- format(date_decimal(x), "%d-%m-%Y"))
# [1] "01-01-1988" "01-04-1988" "02-07-1988" "01-10-1988"

Then you can write it to a csv with

write.csv(f, "afilename.csv")   ## or write.table()

You'll probably want to check the output first and adjust some of the arguments to whatever format you want.

Convert decimal month and year into Date

You may use some nice functions from the zoo package:

as.yearmon to convert year and floor of the decimal month to class yearmon.

Then use as.Date.yearmon and its frac argument to coerce the year-month to class Date.

library(zoo)
df$date = as.Date(as.yearmon(paste(df$year, floor(df$decimal_month), sep = "-")),
                  frac = df$decimal_month - floor(df$decimal_month))

#       decimal_month year        date
# 1              4.75 2011  2011-04-22
# 2              5.00 2011  2011-05-01
# 3              5.25 2011  2011-05-08 

If desired, day of year is simply format(df$date, "%j")

How to convert decimal date format (e.g. 2011.580) to normal date format?

Slightly different results with lubridate:

library(lubridate)
decimals <- c(2001.667, 2004.083, 2008.750, 2011.583, 2011.917)

format(date_decimal(decimals), "%Y-%m-%d")
# [1] "2001-09-01" "2004-01-31" "2008-10-01" "2011-08-01" "2011-12-01"

How to calculate a decimal month in R in a particular year?

You can use monthDaysfunction from Hmisc package

> require(Hmisc)
> library(lubridate)
> month(dates) + day(dates)/monthDays(dates)
 [1] 1.774194 4.266667 3.161290 3.258065 2.500000 1.903226 2.750000 3.903226 4.033333
[10] 2.500000 1.903226 1.612903 3.387097 1.967742 4.566667

Substract decimal years from date in r

We could use years and months

v1 <- 5.5
yr <- as.integer(v1)
mth <- as.integer((v1* 12) %% 12)
ymd("2021-05-21") - (years(yr) + months(mth))
#[1] "2015-11-21"

Dealing with numeric (decimal) dates in R?

If you can use POSIXct, you can do for example

df$Opentime <- as.POSIXct( df$Opentime*24*60*60, 
                origin="1900-01-01", 
                tz="UTC")

Reasoning: POSIXct is just number of seconds since origin

How to find decimal representation of years in R?

It's very unclear what you're trying to do exactly here, which makes accuracy difficult to talk about.

lubridate has a function decimal_date which turns dates into decimals. But since 3 decimal places gives you 1000 possible positions within a year, when we only have 365/366 days, there are between 2 and 3 viable values that fall within a day. Accuracy depends on when in the day you want the result to fall.

> decimal_date(as.POSIXlt("2016-01-10 00:00:01"))
[1] 2016.025
> decimal_date(as.POSIXlt("2016-01-10 12:00:00"))
[1] 2016.026
> decimal_date(as.POSIXlt("2016-01-10 23:59:59"))
[1] 2016.027

In other words, going beyond 3 decimal places is only really important if you're interested in the time of day.

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