Numeric to Date in R Excel Central Time : I cant seem to find the answers to this, I have tried multiple things - I am still very new to R

I have two variables in a dataset "Old Data"
"Open Date" - num from Excel and 
"Closed Date" - POSIXct, format: "2017-10-11 12:39:37"

I can't get them to subtract for each line within the dataset, the conversions seem to be off

This is what I am trying to do (without the syntax
Newdata <- Newtime=(Closed Date-Open Date) from "Old Data"

Thanks in advance!