Verified answer

Do you want to enter a value of 7.30 and have Excel turn it into the time 7:30?

Try using the DOLLARDE() function:

A1 7.30

B1 =DOLLARDE(A1, 60) / 24

Format B1 as a time (e.g. h:mm, [h]:mm). In older versions of Excel, you might have to activate the Analysis Toolpak to gain use of the non-standard DOLLARDE() function.

If you want to convert the other way:

A2 7:30

B2 =HOUR(A2) + MINUTE(A2) / 100

There are ways to do this specifically with VBA, especially if you want the conversion to happen in the same cell.