除了这个限制，好像也不用把旧房子的年限算到免税比例里面，总的来说是省了一些税。
https://www.irs.gov/publications/p523
Eligibility Step 1—Automatic Disqualification
Determine whether any of the automatic disqualifications apply.
Your home sale isn’t eligible for the exclusion if ANY of the following are true.
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You acquired the property through a like-kind exchange (1031 exchange), during the past 5 years. See Pub. 544, Sales and Other Dispositions of Assets.
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You are subject to expatriate tax. For more information about expatriate tax, see chapter 4 of Pub. 519, U.S. Tax Guide for Aliens.
Like-kind/1031 exchange.
If you sold a home that you acquired in a like-kind exchange, then the following test applies.
You can’t claim the exclusion if:
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Either (a) or (b) applies:
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You acquired your home in a like-kind exchange (also known as a section 1031 exchange), or
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Your basis in your home is determined by reference to a previous owner's basis, and that previous owner acquired the property in a like-kind exchange (for example, the owner acquired the home and then gave it to you); and
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You sold the home within 5 years of the date your home was acquired in the like-kind exchange.
