"...all the days ordained for me were written in your book
before one of them came to be."
I am sure glad that God is not locked in to just one kind of speech. He knows what to say and how to say that right thing at the right time to the right person - every time. In Paul's second letter to Timothy, he writes, "All Scripture is God-breathed and is useful for teaching, rebuking, correcting and training in righteousness," (2 Tim. 3:16). This means even our beloved poet David, as he waxes on so eloquently in many Psalms, breathed or penned the Truth.
Here in one of my favorite chapters of the Bible we find one of the most ginormous books you will ever hear mentioned. This book contains everyone's birth through death dates next to their names, much like you see on a tombstone - "all the days ordained for me were written in your book". It gets even more interesting. My life was not stamped at birth, nor thought about as I went along living, nor at the end - my days, everyone's days "were written in [His] book before one of them came to be."
The idea is what we do with time in between start and finish on this earth matters. Not everyone assigns meaning to this life and this is sad. I believe we are in a preparation phase that reaps eternal rewards and benefits. This entire Psalm (Psalm 139) is about the inability to escape God's presence in our lives, therefore the necessity to surrender to His will, plan and governing of it at the end of the Psalm (verses 23-24). Since our days are numbered and our life has bookends why not entrust to God - Everything In Between.