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rdatetime-seriesrecode

How to place dates on the same scale using a reference point

I have a set of dates for multiple years and I am wondering how to place them on the same scale using a reference point.

For example, I have the following dates:

"2018-04-15" "2018-04-30" "2018-05-06" "2018-05-12" "2018-05-13"

I want to create a separate column that counts the number of days these dates are from

"2018-11-06".

Thanks so much!

Solution

  • you can use difftime like this

    library(data.table)

## Create data
df <- data.table(Date = c("2018-04-15", "2018-04-30", "2018-05-06", "2018-05-12", "2018-05-13"))
reference <- "2018-11-06"

## Calculate difference in dates in days
df <- df[,DaysFromRef := difftime(as.Date(reference), as.Date(Date), "days")]
df

         Date DaysFromRef
1: 2018-04-15    205 days
2: 2018-04-30    190 days
3: 2018-05-06    184 days
4: 2018-05-12    178 days
5: 2018-05-13    177 days

## Convert DaysFromRef column to numeric
df$DaysFromRef <- as.numeric(df$DaysFromRef)