Calculating elapsed time for different interview dates in R - r

Question

So my data looks like this
dat<-data.frame(
subjid=c("a","a","a","b","b","c","c","d","e"),
type=c("baseline","first","second","baseline","first","baseline","first","baseline","baseline"),
date=c("2013-02-07","2013-02-27","2013-04-30","2013-03-03","2013-05-23","2013-01-02","2013-07-23","2013-03-29","2013-06-03"))
i.e)
subjid type date
1 a baseline 2013-02-07
2 a first 2013-02-27
3 a second 2013-04-30
4 b baseline 2013-03-03
5 b first 2013-05-23
6 c baseline 2013-01-02
7 c first 2013-07-23
8 d baseline 2013-03-29
9 e baseline 2013-06-03
and I'm trying to make a variable "elapsedtime" that denotes the time elapsed from the baseline date to first and second round interview dates (so that elapsedtime=0 for baselines). Note that it varies individually whether they have taken further interviews.
I tried to reshape the data so that I could subtract each dates but my brain isn't really functioning today--or is there another way?
Please help and thank you.

Answer 1

Screaming out for ave:
I'll throw an NA value in there just for good measure:
dat<-data.frame(
subjid=c("a","a","a","b","b","c","c","d","e"),
type=c("baseline","first","second","baseline","first","baseline","first","baseline","baseline"),
date=c("2013-02-07","NA","2013-04-30","2013-03-03","2013-05-23","2013-01-02","2013-07-23","2013-03-29","2013-06-03"))
And you should probably sort the data to be on the safe side:
dat$type <- ordered(dat$type,levels=c("baseline","first","second","third") )
dat <- dat[order(dat$subjid,dat$type),]
Turn your date into a proper Date object:
dat$date <- as.Date(dat$date)
Then calculate the differences:
dat$elapsed <- ave(as.numeric(dat$date),dat$subjid,FUN=function(x) x-x[1] )
# subjid type date elapsed
#1 a baseline 2013-02-07 0
#2 a first <NA> NA
#3 a second 2013-04-30 82
#4 b baseline 2013-03-03 0
#5 b first 2013-05-23 81
#6 c baseline 2013-01-02 0
#7 c first 2013-07-23 202
#8 d baseline 2013-03-29 0
#9 e baseline 2013-06-03 0

Answer 2

This makes no assumptions that baseline is the always at position 1:
dat$date <- as.Date(dat$date)
dat$elapesed <- unlist(by(dat, dat$subjid, FUN=function(x) {
as.numeric(x$date - x[x$type=="baseline",]$date)
}))