Hindu Panchang Siddhānta and Festival Nirṇaya by Pundit Mahesh Shastriji -- आह्निकनिर्णयम्

Hindu Panchang for the World
Pundit Mahesh Shastriji is the Panchang Siddhanti of mypanchang.com
Panchang, hindu calendar discussion (based on Drishya Ganitha (Drig-Ganita) blog for  mypanchang.com