Parsing Periods

[oxinabox]

It would be useful to have the functionality in Dates for parsing periods.
E.g. getting an anser like

julia>parse(Dates.CompondPeriod, "00:15:00", periodformat"HH:MM:SS")
15 minutes

Or even

```julia
julia>parse(Millisecond, "00:15:00", periodformat"HH:MM:SS")
900000 milliseconds

Having this kind of functionality for working with Periods would have been useful for
JuliaLogging/LoggingExtras.jl#32

[quinnj]

I mean, it's not pretty, but you can do: Dates.Millisecond(Dates.Nanosecond(Dates.value(Dates.Time("00:15:00", "HH:MM:SS"))))

[oxinabox]

you can also do:
parse(DateTime, "0:15:00", dateformat"HH:MM:SS") - DateTime(1)
(note the 1 not 0 cf #38870)

[kcajf]

While I haven't seen it used much, there is also an ISO 8601 format for representing durations / periods: https://en.wikipedia.org/wiki/ISO_8601#Durations.