This is kinda like another question you may have seen before. 🙂
The key issue is: what day should a week number specify? Is it the first day of the week? The last? That’s ambiguous. I don’t know if week one is the first day of the year or the 7th day of the year, or possibly the first Sunday or Monday of the year (which is a frequent interpretation). (And it’s worse than that: these generally appear to be 0-indexed, rather than 1-indexed.) So, an enumerated day of the week needs to be specified.
For instance, try this:
as.POSIXlt("2008 42 1", format = "%Y %U %u")
%u indicator specifies the day of the week.
Additional note: See
?strptime for the various options for format conversion. It’s important to be careful about the enumeration of weeks, as these can be split across the end of the year, and day
1 is ambiguous: is it specified based on a Sunday or Monday, or from the first day of the year? This should all be specified and tested on the different systems where the R code will run. I’m not certain that Windows and POSIX systems sing the same tune on some of these conversions, hence I’d test and test again.