How do I convert date to number of days in R

Question

How do I convert date to number of days, starting from the first day of the year.

How do I convert the following to the expected result below?

   Dat         


        

Answer 1

The %j datetime formatting flag will give you the day of the year starting at 0.

d <- read.table(text='Date
02/01/2000         
20/02/2000         
12/12/2000         
13/01/2001', header=TRUE)

d<-transform(d, NumDays=as.numeric(strftime(as.Date(Date, format='%d/%m/%Y'), '%j'))-1)
#         Date NumDays
# 1 02/01/2000       1
# 2 20/02/2000      50
# 3 12/12/2000     346
# 4 13/01/2001      12

Then to add the TotalDays, you can use cumsum with some modular arithmetic,

transform(d, TotalDays=cumsum(c(1, ifelse(diff(NumDays) > 0, diff(NumDays), diff(NumDays) %% 365 + 1))))
#         Date NumDays TotalDays
# 1 02/01/2000       1         1
# 2 20/02/2000      50        50
# 3 12/12/2000     346       346
# 4 13/01/2001      12       378

Or use this shorter alternative.

transform(d, TotalDays=cumsum(c(1, diff(as.Date(Date, format='%d/%m/%Y')))))