Yahoo CEO Marissa Mayer has a reputation for being late. You really don’t want to be like her.

Some people might say they’re late because they get involved in something and just can’t extract themselves. Or they feel they work crazy hours so they’re exempt from the clock. Business Insider recently reported Marissa Mayer’s lateness is a holdover from the culture at her previous employer, Google.

But I think tardiness is the height of disrespect: whether you’re a founder, tech guru, graphic artist, or salesperson. A late person is downright self-centered and doesn’t give a damn about wasting other people’s time.

I’ve developed strict policies about lateness to make sure no colleague, customer, or supplier is kept waiting—ever. This isn’t just because I think time management is important; it’s because people who ignore time constraints are a drain on my business.
If you or your team members are late….

You’ll lose money.

At my company, Metal Mafia, I have a strict start time; all staff members are to be in their seats and ready to work at 9 a.m. sharp. This means that their coffee is poured, their coats and other items are stowed, and their mind is focused on business.

I think about it like this: If Jim wanders in five minutes late for a meeting with Sally because he stopped for Starbucks on the way, five minutes of Sally’s time has been wasted, and the business loses five minutes during which each employee could have been helping customers. Co-workers need to be able to count on access to their counterparts during defined business hours. Failing to enforce this allows your company to lose productivity, which ultimately causes the business to lose money.

You’ll ruin your image.

When you tell a client or a supplier that you will meet at a certain time, and then you don’t show up at the appointed hour, you put a dent in your credibility.

If one of my account executives were to tell a customer she would call on Tuesday to help with his order and then didn’t call until Wednesday or Thursday, it would be easy for that customer to make the mental leap that an order promised to ship today might not actually go out until next week.
A business that is committed to a customer or partner shows that commitment, first and foremost, by showing up, as agreed upon. Allowing anyone on your team to be less than 100 percent committed to punctuality allows your brand to be tarnished.

You’ll decrease your efficiency.

As a manager, I make sure that my staff members have the right workload. If an employee is not able to complete her tasks in the allotted time, barring some rare extenuating circumstance, she is not well organized. Her projects, colleagues, and customers will all suffer because of it.
A customer who doesn’t get told about a limited-time sale on the first day doesn’t have as much time to shop as someone who found out right away. A call that does not get returned on the same day is an opportunity slipping away or a problem being allowed to fester.

Companies that have staying power are companies that do everything—not just launch products and deliver merchandise (though that’s crucial, too)—in a timely manner.

There is no get-out-of-jail-free card when it comes to being punctual. You either care enough to be on time or you don’t. Excuses are just a way to hide that.