Worksheets("TnxDump").Range("A" & i & ":V" & i).Copy Worksheets("TnxSales").Range("A" & lastrow & ":V" & lastrow)
For dates less than 12th - month and day are interchanged while pasting. Like 06-Aug-2015 is converting into 08-June-2015
Please can you confirm the format of the dates in the TnxDump worksheet.
Also as a starting point take a look here: Excel 2010 - change US dates to UK format