You can't.
1 is a day in Excel. 12 hours is half a day, 0.5.
You want to enter 120000 (which to Excel is 120 000 days, more than 300
years) and suddenly it is 0.5.
Formatting does not change content that way. You need a macro solution.
There's one at Chip's page
http://www.cpearson.com/excel/DateTimeEntry.htm

HTH. Best wishes Harald


"Oonagh" skrev i melding
...
How do I format a column so that when I put in 6 digit numbers they will
automatically format as hh:mm:ss? When using the Format Cells option and
then
Time hh:mm:ss, the digits appear as 00:00:00 when you tab out of the cell.