I consider the ERA already ratified and is presently the 28th Amendment. The date limiting clause is in the resolution submitting the amendment to the States, not in the amendment text itself.
Resolved by the Senate and House of Representatives of the United States of America in Congress assembled (two-thirds of each House concurring therein), That the following article is proposed as an amendment to the Constitution of the United States, which shall be valid to all intents and purposes as part of the Constitution when ratified by the legislatures of three-fourths of the several States within seven years from the date of its submission by the Congress:
“ARTICLE —
“Section 1. Equality of rights under the law shall not be denied or abridged by the United States or by any State on account of sex.
“Sec. 2. The Congress shall have the power to enforce, by appropriate legislation, the provisions of this article.
“Sec. 3. This amendment shall take effect two years after the date of ratification.”
Congress does not have the power to set a date limit directly. Congress only has Article V power to propose amendments, not to dictate how those amendments are ratified. It can make (and has made) a date limit part of the amendment text itself, basically forcing the States ratify the time limit when ratifying the whole text. See the 22nd Amendment, Section 2:
This Article shall be inoperative unless it shall have been ratified as an amendment to the Constitution by the legislatures of three-fourths of the several states within seven years from the date of its submission to the states by the Congress
Congress can say “here’s a new rule to vote on” and part of that rule can be the rules of passing that rule. But it can’t say “here’s a new rule to vote on and here are the new rules to follow when voting on that rule”.