Conversions for DateTime and Duration into SystemTime/Duration

[luben]

Having a way to convert into std::time structs would help, e.g.

impl From<DateTime> for std::time::SystemTime { ...}
impl From<&DateTime> for std::time::SystemTime { ...}
impl From<Duration> for std::time::Duration { ...}
impl From<&Duration> for std::time::Duration {...}

[aminalaee]

If this is accepted, I can pick this up.