This comes up now and again and I always seem to forget how I did it last time, so I am posting this here for safekeeping.
In the code page (Report > Report Properties > Code) add the function:
Function formatSeconds(ByVal secs As Integer) As String
Dim t As TimeSpan = TimeSpan.FromSeconds(secs)
For the Cell that you want to format, call the function:
And the date will be formatted as so - it's nice way to express duration in a more readable way, especially when rows may have tens of thousands of seconds: