Start_Date  Difference  Status
1/1/2018    16  Fail
1/1/2018    16  Fail
1/1/2018    16  Pass
1/1/2018    16  Pass
5/1/2018    12  Fail
5/1/2018    12  Fail
5/1/2018    12  Fail
5/1/2018    12  Pass
10/1/2018   7   Pass
10/1/2018   7   Pass
10/1/2018   7   Pass
10/1/2018   7   Fail

If (df$Status == "Pass") { df$Difference <- "I have not idea" } else { df$Difference <- Sys.Date()-df$Start_Date }