In Noda Time 1.3.1, our .csproj file referred to Profile 328 and our .nuspec file put the results in
lib\\portable-net4+sl5+netcore45+wpa81+wp8+MonoAndroid1+Mono
There's been some issues with Xamarin frameworks in the past, but try doing something like this;
https://github.com/AutoMapper/AutoMapper/blob/93f276fea36dedb2efd861096f881667af880d58/src/AutoMapper/project.json
See if this works:
{
"frameworks": {
".NETPortable,Version=v4.0,Profile=Profile328": {
"frameworkAssemblies": {
...
}
}
}
}