Combining data from two variables in R

I'm new to R and can't seem to find an answer to this really basic problem: I want to create a new variable "time2" in my dataset "mm" using numerical values from two other variables. If the variable "Datediff_doed" (time until death) has a value, I would like time2=Datediff_doed, otherwise (if Datediff_doed = NA) I want it to have the value of another variable "Datediff_end" (end of follow-up).

I've tried with "if...else":

 mm$time2<- if (mm$Datediff_doed>0) {mm$time2=mm$Datediff_doed} 

                                    else {mm$time2=mm$Datediff_end}

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"ifelse":

 mm$time2<- ifelse(mm$Datediff_doed>0,mm$Datediff_doed,mm$Datediff_end)

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and copying Datediff_doed and the overwriting the NA's:

 mm$time2=mm$Datediff_doed

 mm$time2[is.na(mm$Datediff_doed)=T] <- mm$Datediff_end

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But clearly I'm doing something wrong..!

I would be very grateful for some help.

Neel