Here is my solution to produce a date value if your data set only has week values associated with data (ie no date).
It gives the first monday of the week.
Michael Folkes
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s <- seq(as.Date("2010-01-01"), as.Date("2010-12-31"), by = "day") #produce all days of the year
series.mon<-data.frame(day.monday=s[format(s, "%w") == "1"],week.val=as.integer(format(s[format(s, "%w") == "1"],"%W"))) #calc week value for every day

my.data.weeks<-sort(sample(1:52, size = 10, replace = FALSE))  #fictitious weekly data (assuming I don't have the associated date)
data.frame(my.data.weeks, first.weekday=series.mon[ series.mon$week.val %in%  my.data.weeks,1]) #combine


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