Everyone starts the day with a head full of ambition.

You're going to get a lot of things done: write that essay, keep the pilates appointment (for once), get your tax papers in order, bake that cake for mom's birthday.

The trouble is, you might still be dreaming.

There's a fundamental disconnect between what we think we can get done in a day and what's actually possible. That's because, as the alarm clock reminds us every morning, our lives are on the meter.

Think of time in concrete terms

"Time is actually not intangible; it is concrete and measurable," productivity consultant and author Julie Morgenstern explains in, appropriately, Time magazine. "You can compare [organizing time] to organizing a closet. There's a limited amount of space that is oftentimes crammed with way too much stuff that you can possibly fit."

Ironically, there may not be enough time in our entire lives to soak up all the time management strategies touted on the internet. Much of it focuses on slimming down the non-essentials, like watching TV or browsing the internet — also known as relaxing.

But Morgenstern, who has written the bestselling "Time to Parent," has developed a uniquely simple method for managing the day: approach it like a mathematician.

If math isn't your strong suit, particularly first-thing-in-the-morning arithmetic, don't despair. It's a pretty simple formula. When you add a task to your to-do list, roughly estimate how long it should take and subtract that time from what you've got to work with in a day.

People who do the math, Morgenstern says, are time realists — their lists may not be long, but those jobs get done.

Then there are the people who wake up with a head full of steam, frantically filling their to-do list with nary a thought to how long each task will take.

Invariably, they end their days with a heaping to-do-tomorrow list. And the cycle of daily frustration and stress continues.

Change is hard — but it's not impossible

A steering wheel with post-it notes stuck to it. Time optimists cram more in their to-do lists than they can possibly get done in a day. (Photo: Suzanne Tucker/Shutterstock)

Thankfully, Morgenstern doesn't blame poor time management on our nature. We can all change.

"When you get a label, you can feel very channeled, like there's no opportunity for change or development," she tells The New York Times.

So how do you get from time optimist to time realist? Lump the urgent tasks into a handful of tidy buckets to help you keep track of them.

"If you're a writer, your responsibilities probably include ideating stories, researching, writing, editing, interviewing," Morgenstern tells Time. "Everyone has three to five buckets. You have to make it simpler. The brain can't keep track of more than that."

Next step: get your math on and assign a time for every task.

To-do items that float freely outside of time could haunt you for months. Instead tack everything to a schedule.

"A 'to-do' not connected to a 'when' rarely gets done," she explains. "A to-do requires a certain amount of time in your day. If it's just sitting on a list and you're just waiting for time to materialize, it's not going to."

About the only thing Morgenstern doesn't want us to classify is ourselves. There's no workaholic type in her theories. Or procrastinator or downright slacker box that we can't break out of.

There are just people who do the math. And people who don't.

So maybe the first thing on our to-do list every morning should be a little math.

How to make a to-do list that actually gets done
Looking to check things off your to-do list? Start with a little math.