“Sha la la la la la live for today
And don't worry 'bout tomorrow, hey“

Similar but not exact:

Let sleeping dogs lie.

Here's another good one:

"Don't stop thinking about tomorrow....
....Yesterday's gone, yesterday's gone"

-F.M.

“Ardently do today what must be done. Who knows? Tomorrow, death comes.”

However, reason suggests the same.

"It doesn't matter" doesn't really match. It might matter.

...and would argue in the final analysis that both are saying, in effect, that only the day(s), within which I exist, matter(s).

The Greeks merely go one step further and exclaim: beyond that, I have no care.

Non curo.

"Desire not the results of today's actions (karmas) that of their own nature will manifest reactions (karma phala), equal to the bad (and good) done."

Take therefore no thought the results of today's actions, thought for the things of itself (nature of things) will manifest reactions (the morrow), sufficient (equal) to the evil done today.

Bhagavad Gita 2:47 (variously translated from Sanskrit)

You have a right to perform your prescribed duties, but you are not entitled to the fruits of your actions. Never consider yourself to be the cause of the results of your activities, nor be attached to inaction.

You only have authority over action, not ever to the result. Don’t be motivated by the result, and don’t get attached to inaction.

You have the right to work only but never to its fruits. Let not the fruits of action be your motive, nor let your attachment be to inaction.

Your right is to perform your work, but never to the results. Never be motivated by the results of your actions, nor should you be attached to not performing your prescribed duties.

Simplify.

That's my interpretation of that verse.

More like always only this one day, as the morrow never comes.

Yesterday cannot be changed. Tomorrow is uncertain.

The only thing I knew how to do
Was to keep on keepin’ on like a bird that flew
Tangled up in blue

Or as John Lennon once said "Life is what happens when you're making other plans"

'be best' slogan by rump's wife.

"Eat, drink, and be merry, for tomorrow we may die," which doesn't quite fit the sentiment, I know.

"Don't sweat the small stuff," likewise seems appropriate.