If you're just changing the format, then it's probably viewing them as Julian dates is my guess.
What you need is a sort of complex formula(or in other words, functions embedded in a main formula, or to do it in VBA.
Do you have any preference? I think the best way would be to do it in VBA, and if it's something you do all the time, you could create your own custom Formula in VBA, and then just reference that in the Excel cells.
Like this: Cell A1 - enter "=MyNewFormula(CellReferences)"
If you want to try any of the VBA routes, then you might want to move your question over to forum707.
If not, then we can go on with getting the formula right in Excel. I think you can use a few formulas such as IIF, Left, Right, etc to get the right splits, but I think it'll be quite a long formula.
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"If to err is human, then I must be some kind of human!" -Me