Adding a date to a string Message

I am trying to create an error message based on a time frame, the last 15 min. and now. So the error message would say, "Client Missed file between 15:15:00 - 15:30:00" The times are calculated at the time of the search and the following search below fails as "Error in 'eval' command: Typechecking failed. '+' only takes two strings or two numbers." | eval 15MinEarly=strftime(relative_time(now(), "-15m"), "%m/%d/%Y %H:%M:%S") | eval Now=strftime(now(), "%m/%d/%Y %H:%M:%S") | eval ErrorMessage = "Client Missed file between: " + 15MinEarly + " - " Now How do you convert the two times to string so I can concatenate them into the error message?