A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
The only way I can think of doing this is via Control Panel > Region and Language > Additional Settings > Time and entering HH.mm as the short time format and HH.mm.ss as the long time format.
You can then enter times as 08.15.00 or 08.15. but not 08.15 since that is taken to be a number.
Making such a global change would be unwise, it seems to me, as it might break other software.
An alternative is just to enter time as e.g. 815 or 1720 and then run a macro to turn these integer values into time values. See for example: