I've searched and experimented and cannot find a solution to this problem. Using Excel 2010.

I have raw time data, formatted as General, from a database that represents mm.ss.0 (tenths of seconds), with the value of the minutes being very large. Example: 994369:17.3. I need to SUM many rows of time and prefer to stay in the same format and not be in the hh:mm:ss format. I've formatted the cells in many different ways and each time the SUM result is 00:00.0.

Here are examples of the data I am working with and trying to add.
2631:18.0
547:17.5
280986:52.0
994369:17.3
659639:00.6