Sikhí is a Dharm, a way of life, Sikhí is not a doctrinal system. Guru writes about hell and heaven and about the cycle of birth and death, Guru uses ‘Hindu’ language and ‘Muslim’ language without any compunction.
Guru does not say : There is no Hindu, no Musulman, become a Sikh. Guru says be a good Hindu, be a good Muslim. The Guru wants all to be sikhs with a small s, students of God. Guru wants all to follow the path of righteousness, the path of honesty, compassion, of seeing God in all, of serving all. Guru wants us to discover God’s never ending love for all creation, and to develop our love for Him/Her.
This makes the various debates going on at the moment so unreal. Calendars are not good or bad because they are Christian, Hindu or Sikh, a calendar should be a correct and useful tool to tell days, weeks, months, seasons and years apart.
We should serve God and God’s creation every day of every calendar. I think that the Nanakshahi calendar is a useful tool and I think that following the Vikramí calendar serves no useful purpose. But it is not a matter of life and death for the Sikh community.
You can follow Guru’s way of righteousness while using the Jewish, the Muslim, the Christian, the Nanakshahi, the Vikramí or any other calendar.
The other issue, the discussion about the Dasam Granth, makes even less sense. Guru Gobind Singh clearly told us to follow the Guru Granth Sahib. Guru Gobind Singh made no attempt either at Talwandi Sabo (Damdama Taksal) or in Hazur Sahib to put together another Granth.
All this does not mean that I pretend to know who was/were the author(s) of the Dasam Granth. Even if there was no doubt whatsoever that all of the Dasam Granth was written by Guru Gobind Singh, the above would apply, by the hukam of Guru Gobind Singh himself.
We are a liberal tradition, we can read any book, whether from the Sikh tradition or from any other faith. But the Guru Granth is our Guru and we should ‘measure’ all other books against the teachings of our Granth.
I am not against further research into the Dasam Granth and its author(s) as long as this does not stop us from reading and understanding the Guru Granth Sahib.
Finally, threatening people with violence because they disagree with you is despicable by anybody’s standard. Those that threaten violence or use violence against Sikhs because of disagreements clearly go against Gurmat.