You can probably check an individual player's buffs if you are looking for the Skull Banner buff. It will tell you the uptime and the number of time that player gained that buff. If you then hit the hashtag on the right-hand side of that buff area, you can see when it was cast. In case it's not clear:
1) Go to the fight you want to analyze.
2) Click on a player.
3) Select the "buffs gained" tab.
4) You win!