How to count the days spent in US

Are you counting days for the substantial presence test? If so, days you commute to work in the US don't count. You would not be able to use Sunday.

You are treated as present in the U.S. on any day you are physically present in the country, at any time during the day. However, there are exceptions to this rule. Do not count the following as days of presence in the U.S. for the substantial presence test:

• Days you commute to work in the U.S. from a residence in Canada or Mexico if you regularly commute from Canada or Mexico.
• Days you are in the U.S. for less than 24 hours, when you are in transit between two places outside the United States.
• Days you are in the U.S. as a crew member of a foreign vessel.
• Days you are unable to leave the U.S. because of a medical condition that develops while you are in the United States.
• Days you are an exempt individual

Substantial Presence Test

Thank you for the reply. I'm a bit confused with the first exception

• Days you commute to work in the U.S. from a residence in Canada or Mexico if you regularly commute from Canada or Mexico.

For my case, I think Sunday should not be counted for substantial presence test. If I left in the early Monday morning, drive 2 hours to the office and start to work on Monday. Should Monday be counted?

If you arrive in the US before midnight Sunday then Monday would be counted as being present in the US. One Monday you are commuting from a US location, not from Canada.

I think there is a bit misunderstanding here. My question is if I left Canada at 6 am on Monday, drive 2 hours in US and arrive US office at 8 am Monday, then start to work at 8 am Monday. Because on Monday I commute to work and work on Monday. Will this Monday count?

Monday would not count as a day or presence towards the substantial presence test.  See IRS Publication 519 page 4.

Regular commuters from Canada or Mexico.

Do not count the days on which you commute to work in the United States from your residence in Canada or Mexico if you regularly commute from Canada or Mexico.