Datetime parse format for round-trip "O"

[treeform]

Seconds fractions are not commonly supported or found in the wild, but you can 
still parse them with my calendar/timestamp/timezone library:

github: <https://github.com/treeform/chrono> docs: 
<https://nimdocs.com/treeform/chrono/chrono.html#format-specification>
    
    
    import chrono
    
    let cal = parseCalendar(
      
"{year/4}-{month/2}-{day/2}T{hour/2}:{minute/2}:{second/2}.{secondFraction}Z",
      "2022-04-25T13:03:28.8936219Z"
    )
    
    echo cal
    echo cal.second
    echo cal.secondFraction
    
    
    Run

Just `nimble install chrono` to get it.