From what I've gathered, the modern dating system is indeed post-Lanival. That much is evidenced by the fact that most of the months are named for his war buddies, and our years are referred to as "AV" or "AL" which mean After the Victory and After Lanival, respectively. AV makes more sense, since our years don't actually start after Lanival split up the provinces but rather after the Dragon's Empire fell, but you will see both.
Pre-Lanival stuff, the years tend to be referred to as "IY" which means "Imperial Year" IIRC. I have no clue what they called the months, or at what point their dating system begins.
I might be wrong on that stuff too, but I'm too lazy to check.
Rev. Reene
You whisper to Ventuul and Szrael, "OOC: I'm bored. you guys up for some fun?"
Szrael whispers to your group, "OOC: Thus began the great war between zoluren and qi'reshalia"
