So far, in Julia converting a CompoundPeriod type object to any other Period type or TimeType is not straightforward. @Laco_Kovac, you can notice another topic here.
@quinnj, I think you are trying to say something like this:
julia> using Dates
julia> x = Dates.CompoundPeriod(Dates.Hour(12), Dates.Minute(13))
12 hours, 13 minutes
julia> Dates.format(Dates.Time(x.periods[1], x.periods[2]), dateformat"HH:MM:SS")
"12:13:00"