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...blem before calculating age in the code below? DOB starts out as a factor in the Demo dataframe but then is converted into a date. So I had thought the ifelse that follows could be used to correct the problem, but this doesn't seem to be the case. Thanks, Paul Demo_Char <- within(Demo, { DateCompleted <- as.Date(DateCompleted, format = "%m/%d/%Y") DOB <- as.Date(DOB, format = "%m/%d/%Y") DOB <- ifelse(Subject==108945, as.Date("1931-11-23"), DOB) Age <- as.integer((DateCompleted - DOB) / 365.25) })