Packing a Timestamp into a Protobuf3 struct

Question

I am currently trying to use protobuf for data exchange.

My message needs to have a dynamically typed dictionary to transfer data so I am using the protobuf type Struct.

I am now trying to solve the problem that Struct only seems to work with primitive types but I need to at least be able to also give it a Timestamp value.

Protofile:

syntax = "proto3";

import "google/protobuf/timestamp.proto";
import "google/protobuf/struct.proto";

message SensorReading { 
    enum SensorTypes {
        Generic_MultiValue = 0;
        GPS = 1;
    }
    google.protobuf.Timestamp MeasurementTime = 1;
    string SensorName = 2;
    SensorTypes SensorType = 3;
    google.protobuf.Struct Values = 4;
}

C# code:

using ProtoTypes = Google.Protobuf.WellKnownTypes;
using Google.Protobuf;

(...)

SensorReading reading = new SensorReading()
{
    MeasurementTime = ProtoTypes.Timestamp.FromDateTimeOffset(DateTimeOffset.UtcNow),
    SensorName = "me3630_gps",
    SensorType = SensorReading.Types.SensorTypes.Gps,
    Values = new ProtoTypes.Struct()
    {
        Fields =
        {
            ["positionTime"] = ProtoTypes.Timestamp.FromDateTimeOffset(gps.PositionTime), // <--- Timestamp is not compatible with Value
            ["latitude"] = ProtoTypes.Value.ForNumber(gps.Latitude),
            ["longitude"] = ProtoTypes.Value.ForNumber(gps.Longitude),
            ["speedOverGround"] = ProtoTypes.Value.ForNumber(gps.SpeedOverGround),
        }
    }
};

Is it possible to somehow pack timestamp into a struct or will I have to use a workaround? (For example use a number and convert the time to a unix timestamp)