https://www.law.cornell.edu/uscode/text/26/1033

"If the taxpayer during the period specified in subparagraph (B), for the purpose of replacing the property so converted, purchases other property similar or related in service or use to the property so converted,"

"The period referred to in subparagraph (A) shall be the period beginning with the date of the disposition of the converted property, or the earliest date of the threat or imminence of requisition or condemnation of the converted property, whichever is the earlier, and ending—

(i) 2 years after the close of the first taxable year in which any part of the gain upon the conversion is realized, or"

Pertinent parts. You can read the whole part at that link.