Time_df =
                  Time1               Time2      Time3      Time4      Time5
102 1970-01-01 07:00:00 1970-01-01 19:00:00 1970-01-01 1970-01-01 1970-01-01
103 1970-01-01 00:00:00 1970-01-02 00:00:00 1970-01-01 1970-01-01 1970-01-01
104 1970-01-01 00:00:00 1970-01-02 00:00:00 1970-01-01 1970-01-01 1970-01-01
105 1970-01-01 06:00:00 1970-01-01 22:00:00 1970-01-01 1970-01-01 1970-01-01
106 1970-01-01 08:00:00 1970-01-01 21:00:00 1970-01-01 1970-01-01 1970-01-01

str(Time_df)
'data.frame':   5 obs. of  5 variables:
 $ Time1: POSIXct, format: "1970-01-01 07:00:00" "1970-01-01 00:00:00" "1970-01-01 00:00:00" ...
 $ Time2: POSIXct, format: "1970-01-01 19:00:00" "1970-01-02 00:00:00" "1970-01-02 00:00:00" ...
 $ Time3: POSIXct, format: "1970-01-01" "1970-01-01" "1970-01-01" ...
 $ Time4: POSIXct, format: "1970-01-01" "1970-01-01" "1970-01-01" ...
 $ Time5: POSIXct, format: "1970-01-01" "1970-01-01" "1970-01-01" ...

times = strftime(Time_df, format="%H:%M:%S")
Error in as.POSIXlt.default(x, tz = tz) :
  do not know how to convert 'x' to class “POSIXlt”

Mon = rep("03/20/17", 6)
Tue = rep("03/21/17", 6)
Wed = rep("03/22/17", 6)
Thu = rep("03/23/17", 6)
Fri = rep("03/24/17", 6)
Sat = rep("03/25/17", 6)
Sun = rep("03/26/17", 6)

dates = c(Mon, Tue, Wed, Thu, Fri, Sat, Sun)
t = strftime(Time_df, format="%H:%M:%S")
x = paste(dates, t)