Historically, Microsoft had their own timestamp format called Authenticode. From Windows 7 onwards, Microsoft started to support the RFC 3161 Time-Stamp Protocol (TSP). When signing code, supported by both Windows XP and Windows 7 (or Vista), the only viable option was to use the MS Authenticode code-signing technology. While Microsoft still supports both timestamp protocols today, Entrust discontinued the Entrust Authenticode server (http://timestamp.entrust.net/TSS/AuthenticodeTS).
Since Visual Studio 2015 only supports signing applications in the Authenticode TS format, the only viable option is to upgrade to latest Visual Studio version. The URL to Entrusts RFC3161 compliant timestamp server is http://timestamp.entrust.net/TSS/RFC3161sha2TS