Adding lead zeros to time

Hello...

I'm pasting data from an external source into Excel. The data is in hh:mm:ss format. However, when the source data has zero hours it only displays minutes and seconds preceded by a colon. For example, 5 cells of data in a column might look like this:

:29:28
:31:50
1:15:17
:05:47
1:19:36

I need to run simple calculations on these times, but the cells without an hours value paste into Excel as text format. What are some ways to add a zero before the colon in Excel?

Example attached.

Thanks all!