Here's what the IRS says about Temporary Job Assignments in the Publication 463:

Temporary Assignment or Job

You may regularly work at your tax home and also work at another location. It may not be practical to return to your tax home from this other location at the end of each work day.

Temporary assignment vs. indefinite assignment.

If your assignment or job away from your main place of work is temporary, your tax home does not change. You are considered to be away from home for the whole period you are away from your main place of work. You can deduct your travel expenses if they otherwise qualify for deduction. Generally, a temporary assignment in a single location is one that is realistically expected to last (and does in fact last) for one year or less.

However, if your assignment or job is indefinite, the location of the assignment or job becomes your new tax home and you cannot deduct your travel expenses while there. An assignment or job in a single location is considered indefinite if it is realistically expected to last for more than one year, whether or not it actually lasts for more than one year.

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