You could read other content of mine and upvote that. Or you could simply respond to comments of yours within 7 days, if you don't have time upvote them or another post of the Author's. Responding to a post after 15 days and saying you cannot upvote it, is part of the problem, not the solution. I don't mean to be rude, but it is rare that someone replies after a 7 days. The 7 day limit is reasonable.