Don't have a clue, but if you live in Connecticut (06xxx) you run into the
same problem. Highlight the cell, goto Format> Cell>Number>Custom, in the
box labeled Type, enter 00000 (five zeros), then OK. Excel should display
the cell with a five digit number with a leading zero. Not sure, but if you
enter only the four significant digest, Excel may auto add the leading zero.
You can also format an entire row or column the same way.