Well, thinking about Base10 and Base60 logarithms at the moment! Not getting too far!
The quick fix would be the following MACRO:
Sub ReplaceDecimal()
'
' ReplaceDecimal Macro
'
Application.CommandBars("Stop Recording").Visible = False
Cells.Replace What
".", Replacement
":", LookAt
xlPart, SearchOrder _
xlByRows, MatchCase
False, SearchFormat
False, ReplaceFormat
False
End Sub
And run this after your data entry.
This will replace . with : and Excel will understand the time values.
Then as advised above make sure you format the SUM cell as num,ber format [H]:mm
Will kep working and let you know how I get on.