ISO TC 184/SC4 Official Web Site
Science, Reference, Standards, Individual Standards - ISO TC 184/SC4 Official Web Site. SC4 is the ISO submcommittee responsible for the development ISO 10303 and related standards.
http://www.tc184-sc4.org/
HyTime Users' Group Home Page
Science, Reference, Standards, Individual Standards - HyTime Users' Group. Detailed information on many aspects of this standard, with many useful links.
https://www.hytime.org/