[R] Extract time from irregular date and time data records

[Sam Albers]

Hello,

I am having a problem making use of some data outputted from an
instrument in a somewhat weird format. The instrument outputs two
columns - one called JulianDay.Hour and one called Minutes.Seconds. I
would like to convert these columns into a single column with a time.
So I was using substr() and paste to extract that info. This works
fine for the JulianDay.Hour column as there are always five characters
in an entry. However in the Minutes.Seconds column any leading zeroes
are dropped by the instrument. So if I use substr() to selected based
on character position I end up with incorrect times. So for example:

## df

df<-structure(list(Temperature = c(18.63, 18.4, 18.18, 16.99, 16.86,
11.39, 11.39, 11.37, 11.37, 11.37, 11.37), JulianDay.Hour = c(22610L,
22610L, 22610L, 22610L, 22610L, 22611L, 22611L, 22611L, 22611L,
22611L, 22611L), Minutes.Seconds = c(4608L, 4611L, 4614L, 4638L,
4641L, 141L, 144L, 208L, 211L, 214L, 238L)), .Names = c("Temperature",
"JulianDay.Hour", "Minutes.Seconds"), row.names = c(3176L, 3177L,
3178L, 3179L, 3180L, 3079L, 3080L, 3054L, 3055L, 3056L, 3057L
), class = "data.frame")

## Extraction method for times
df$Time.Incorrect <- paste(substr(df$JulianDay.Hour, 4,5),":",
                 substr(df$Minutes.Seconds,1,2),":",
                 substr(df$Minutes.Seconds,3,4),
                 sep="")


## Manual generation of desired time
df$Time.Correct <- c("10:46:08",
"10:46:11","10:46:14","10:46:38","10:46:41","11:01:41","11:01:44","11:02:08","11:02:11","11:02:14","11:02:38")


## Note the absence of leading zeroes in the Minutes.Seconds leading
to incomplete time records (df$Time.Incorrect)
df

##

So can anyone recommend a good way to extract a time from variables
like these two? Basically this is subsetting a string issue.

Thanks in advance!

Sam