So it turns out that some of my date values were returning the error string #N/A. As soon as I corrected the underlying values, I was able to see the correct timestamps.
I have the same problem but I don't have any error messages. My excel sheet has four columns, Date (in date format dd/mm/yy), time (time format 03:00:00) and then two files of numeric data.
I can convert the date and time to strings and then add them in a calculation to give me dd/mm/yy hh:mm:ss and then I can plot my data. The only problem is that I cannot easily et up a filter to plot selected time periods.
Can you please tell me how you resolved your issue? I can then try to replicate that.
Attached is a few lines from the excel file.