David (who wrote this) isn't morbid or obsessing about his life span. David is asking God for perspective. David understands that life is never how he perceives it and wants clarity. Today is not meaningless. Tomorrow is not the most important thing. Yesterday isn't as critical in formulating future as it may appear. However ... Today has a lot of meaning. Tomorrow is important. Yesterday can teach us many things. God's perspective is always much more accurate than ours.
Lord, help me to see the importance of today and how it has been impacted by yesterday but not dictated by it and help me see the importance of tomorrow but not obsess about it so much that I miss out on blessings of today.