Let me answer your comment first. For that modify code like this:
BASICDATETIMEOFINVOICE = "1-Jan-2014";
DateTime date = DateTime.ParseExact(BASICDATETIMEOFINVOICE, "d-MMM-yyyy", CultureInfo.InvariantCulture);
And for your question,code should go like this:
DateTime date =DateTime.ParseExact("1-Jan-2014 16:18","d-MMM-yyyy HH:mm",CultureInfo.InvariantCulture);
Important to refer :
http://msdn.microsoft.com/en-us/library/8kb3ddd4(v=vs.110).aspx[
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Regards..