Check out example codes for "Exception thrown: 'System.FormatException' in mscorlib.dll dates". It will help you in understanding the concepts better.

Code Example 1

/* Just putting this here for myself, but maybe it will help someone else too:

If this error occurs when you are importing data from Excel, 
it is possible that your computer's regional settings are incorrect.

Try using ExactParse if in C# , and if there are still issues it may be the
Excel/ regional setting combo causing the issue.
    formattedDate = DateTime.ParseExact(dateString, "dd/MM/yyyy", null).ToString("yyyy-MM-dd");
  // Might be the wrong expected format, might be regional settings

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