r - Dplyr mutate not applying calculation to all rows in data frame -

i having issue mutate function. trying difference between 2 dates , place in column further analysis. pulled dates csv file , imported strings.

using code:

df%>%     rowwise()%>%     mutate(df,age_when_trade = ifelse(flag1 == true && flag2 == true,as.date(date1,"%m/%d/%y") - as.date(date2,"%m/%d/%y")))

i have been getting below error

error: impossible replicate vector of size 13

i using rstudio 0.98.978 , r version 3.0.2

i able answer taking different route

age<-as.date(df$date1,"%m/%d/%y") - as.date(df$date2,"%m/%d/%y")  df<-data.frame(df,age)

WIKI