Coding Date/Time Custom Format

Is it possible to Format the Date time to read 4-Jun-15 16:00:00 instead of 6/4/15 16:00:00

I don't see this as an option to format cells but could it be coded?

Example:

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I have put this code into the worksheet but the column formatting is over-riding this formatting. How do I make this formatting take control

You can format the cell as a custom format and put in "d-mmm-yy hh:mm:ss"