How to say "Exactly" in Chinese?
In your scenario, I would say "今天正好是你订婚一周年"

正好 
Exactly, and you can also say 正正好好 'absolutely exactly'(don't know how to say it)



This is a common word which can be proved by 

李同志依了好婆的话去做,果然,正正好好的到了头。(文匯月刊, 第 111-115 卷 第25页)

太晚了人家也要怪的,五分钟正正好好, (新华文摘 第 7-12 期 第86页)

正正好好戴在唐僧的脑袋上 (另眼看四大名著: 西遊記、水滸傳、三國演義、紅樓夢
作者：馬亞麗)