You're right: in a sequence diagram, time is understood to pass from the top to the bottom. If you want to change it and make time pass from the bottom up, you will have to put the swimlane's labels at the button, or you'll have issues with swimlane creation if one happens during the sequence (i.e. a message creates a new object with its own swimlane).
Similarly, activity diagrams are understood to be represented with time moving from top to bottom or from left to right.