Long thread, not sure if mentioned.
Anyways, cancelling a guarantee got me a truce lasting 8 years. Not exactly sure what the mechanism was, but basically I beat somebody up, took a bunch of provinces and got a decade long truce. So, to save those provinces for my own taking, I placed a guarantee on my target. When the truce was about one year away from ending, I dropped the guarantee as that used to give you a year long truce. However, in my case it seemed to "reset" the original truce to 8+ years so I ended up unable to attack my chosen target for nearly 20 years.
Anyways, cancelling a guarantee got me a truce lasting 8 years. Not exactly sure what the mechanism was, but basically I beat somebody up, took a bunch of provinces and got a decade long truce. So, to save those provinces for my own taking, I placed a guarantee on my target. When the truce was about one year away from ending, I dropped the guarantee as that used to give you a year long truce. However, in my case it seemed to "reset" the original truce to 8+ years so I ended up unable to attack my chosen target for nearly 20 years.