Is there (one) song that can start your day and...end your day?

For me, yes. It's "Lady Love" by Robin Trower.

There is the positive, overriding aspect that this song gives, then at days' end, the feeling of completion.

How is this possible? Well, after all, it's RT!
'Dead' by Napalm Death.
For those about to Rock. I rock to work and I rock after work, Imma Rocker!
Eels "Wonderful, Glorious"
Louis Armstrong - What a Wonderful World
Katrina and the Waves,"I'm in walking on sunshine".

Cat Power, Amazing Grace
I like to end the night with "Rainy night in Georgia" by Shelby Lynne or "Chimes" by KT Tunstall.
Frontin' - Jamie Cullum.