C# Driver - manually calling Deserializer

I have created a serializer, 'BsonPrettyTimeSpanSerializer', which I use on serialized elements as such;

[BsonSerializer(typeof (BsonPrettyTimeSpanSerializer))]

public TimeSpan ChunkStartAge { get; private set; }

I would like to write some Unit tests for this, but I am struggling to determine how. I was looking at the Version Tests in C# repo;

var version = BsonSerializer.Deserialize<Version>(json);

I am trying to achieve this as such:

var result = BsonSerializer.Deserialize<TimeSpan>(json);

I am concerned, however, that if I have another serializer registered for TimeSpan, it may not select the correct one for my test.

What I would effectively like to know, is how to Deserialize a string with a pre-determined Serializer. How can I achieve this?

Minor update - the Serializer takes a string in format "1d 2h" and converts it to a TimeSpan, and vice versa