Upvote:0
I'd be sceptical of an online resource unless they'd really done their homework. If you want to take up the challenge then you need the standard reference book: "A Handbook of Dates for Students of British History" by Cheney and Jones.
Upvote:2
Dick Henry provides a Julian calendar year-at-a-glance view going back to 46 B.C. on his website, but it abides by the timing specified by the papal bull "Inter gravissimas", so it doesn't show you O.S. dates after A.D. 1582.
Petko Yotov provides a side-by-side year-at-a-glance view extending into the future, going back to A.D. 1582, and providing a Julian calendar year-at-a-glance back to A.D. 1.