Maintaining a leading zero requires either customs formatting or data entry
as text.
Taking the latter first, entering ' as the first digit causes Excel to
recognise data as text. Example '01 will show as 01. Disadvantage: no
mathematical calculations.
You can highlight the column, Right-click > FormatCells > NumberTab > choose
the TEXT selection > OK...........this way, if you have a lot of entries to
do, you don't have to type a leading apostrophe to change each cell to TEXT
format individually..........