The Hardest Math in the Bible

It's the only psalm Moses ever wrote. And it's the one most of us need.

Moses had just buried his sister.

Numbers 20 doesn’t make much of it. Seven words in English — And Miriam died there and was buried. No eulogy. No reflection. Just a sentence dropped into the chronicle like a stone into still water.

Then Moses lost his temper at the rock. He struck it twice when God had told him to speak to it, and he was told he would not be entering the land he’d spent forty years walking toward. One strike away. Now the door was closed. A few verses later, he climbed Mount Hor with his brother Aaron, watched the priestly robe transferred to Aaron’s son, and came back down alone.

Three losses in one chapter. Sister. Vocation. Brother. All inside a stretch of weeks. Many commentators think it was around that moment that Moses sat down and wrote the psalm we know as Psalm 90. It’s the only psalm in the whole Bible attributed to him, and likely the oldest one in the book. What’s striking is what’s not in it. No bitterness. No defeat. No bargaining. Just an old man, with too many graves behind him, doing something most of us never quite get around to.

He starts counting.

An Old Man’s Math

Lord, you have been our dwelling place in all generations… You return man to dust, and say, “Return, O children of man.” For a thousand years in your sight are but as yesterday when it is past, or as a watch in the night… You sweep them away as with a flood; they are like a dream, like grass that is renewed in the morning: in the morning it flourishes and is renewed; in the evening it fades and withers.

— Psalm 90:1, 3-6

Moses had lived in that land long enough to know the picture in those verses. A spring rain comes in the night, and by morning the desert is carpeted in green. By midday, the grass is already fading. By nightfall, it’s brown. One day, start to finish. That’s life, Moses says.

And then, like a man who’s done his sums, he says the number out loud:

The years of our life are seventy, or even by reason of strength eighty…

— Psalm 90:10

Eighty years. About 29,000 days. Most of us never think of life in days. We think in calendars and decades, and one day when I… But Moses is sitting with the unflinching number. If you've ever counted your own — and you can, it's a quick calculation — you know the number lands differently on paper than it does in your head. We round up. We treat days like we'll be issued more. Moses doesn’t.

And that’s where the most important verse in the psalm lands.

The Prayer Underneath the Counting

So teach us to number our days that we may get a heart of wisdom.

— Psalm 90:12

Moses doesn’t pray let me count my days. He prays teach me to count them. There’s a difference, and we need to see it.

You can count days and learn nothing. People do it every December. They look at the year, grieve what they wasted, and announce what they’ll do differently. Then by February, the calendar is full of the same things again. Counting alone doesn’t produce wisdom. Sometimes it produces panic. Sometimes, the I’m running out of time instinct drives a person into the worst decisions of their life: the affair, the impulsive resignation, the long-postponed selfishness finally cashed in. Midlife crises aren’t a failure of math. They’re a failure of teaching.

What Moses prays for is something a calendar can’t give: a heart of wisdom. To know what these days are for. To use them rather than spend them. James Boice once called this “the hardest math in the Bible.” We count calories. We count steps. We count notifications, dollars, likes, and miles. We do not seem to be able to count what actually matters.

That’s why Moses doesn’t just count. He asks God to teach him to.

Establish the Work of Our Hands

There’s one more line in the psalm we need to see. It comes at the very end.

Let the favor of the Lord our God be upon us, and establish the work of our hands upon us; yes, establish the work of our hands.

— Psalm 90:17

He says it twice. Establish the work of our hands. Yes — establish the work of our hands.

You can hear it, can’t you? An old man, sin behind him, regret behind him, a generation he loved buried in the wilderness, his brother gone, the land of promise visible from a mountain he won’t be allowed to enter. And what he wants — what he asks for, twice, like he’s begging — is that the things he did with his small handful of days would last beyond him.

That’s the prayer of a man who has finally numbered the days. Don’t let me spend my life on what dies with me. Establish what I do for You.

Moses didn’t see how that prayer got answered. But you have. The man who thought he’d failed in the wilderness wrote the first five books of the Bible you carry. He raised up a nation. He left a record of leadership that has fed God’s people for thirty-three centuries.

God established the work of his hands.

What Day Is It?

If you do the math, today is some specific number of your days. You could figure it out in about ten seconds. (For me, AI says today is the 19,091st day of my life.)

But Moses’ prayer isn’t for the panicked. It’s for the rested. For the ones who are ready to stop counting alone and start asking God to teach them what these days were given for.

The wisdom isn’t in knowing the number.

The wisdom is in handing the number to Him.