Year-month-week expression

Question

I have a data written in specific expression. To simplify the data, here is the example I made:

df<-data.frame(date=c(2012034,2012044,2012051,2012063,2012074),
               math=c(100,100,23,46,78))

2012034 means 4th week of march,2012. Likewise 2012044 means 4th week of April,2012. I was trying to make the values of date expressing some order. The reason why I have to do this is because when I don't change them to time expressions, x axis of the scatter plot looks really weird.

My goal is this: Find the oldest date in date column and name it as 1. In this case, 2012034 should be 1 . Next, find the second oldest date in date column and calculate how many weeks passed after that date . The second oldest date in date is 2012044 .So, 5 weeks after the oldest date 2012034 . So it should be changed as 1+5=6. So, likewise, I want to number the date to indicate how many weeks have passed since the oldest date

Answer 1

One way to do it is by also specifying the day of the week and subtract it at the end, ie

as.Date(paste0(df$date, '-1'), '%Y%m%U-%u') - 1
#[1] "2012-03-22" "2012-04-22" "2012-05-01" "2012-06-15" "2012-07-22"