FleeceTimeSpan

[<AutoOpen>]
module JsonExtensions = 

    open System
    open Fleece
    open Fleece.Operators
    open FSharpPlus
    open System.Globalization
    
    type FromJSONClass with
        static member ToJSON(x: TimeSpan) = 
            let s = x.ToString("d\.hh\:mm\:ss", CultureInfo.InvariantCulture)
            JString s
        static member FromJSON(_: TimeSpan) =
            function
            | JString s ->
                if s = null
                    then Failure "Expected TimeSpan, got null"
                else match TimeSpan.TryParseExact(s, "d\.hh\:mm\:ss", CultureInfo.InvariantCulture) with
                     | true,ts -> Success ts
                     | _ -> failwithf "Invalid TimeSpan %s" s
            | x -> Failure (sprintf "Expected JString, found %A" x)

https://gist.github.com/mausch/a8012001888e1427c2f1