.NET SDK - AdminToken() generates 'invalid grant'

Using .NET Core, following the sample code.  This part works fine without errors:

config = ConfigureBoxApi();
session = new BoxJWTAuth(config);

But as soon as I call AdminToken() on 'session' I get an 'invalid grant' error:

var adminToken = session.AdminToken();

generates:

{"error":"invalid_grant","error_description":"Current date\/time MUST be before the expiration date\/time listed in the 'exp' claim"}

Since I can't manipulat ethe 'exp' claim, I'm stuck here.

Any solutions?