The IRS knows:

https://www.irs.gov/pub/irs-pdf/i709.pdf

"For each of the 5 years, you report in Part 1 of Schedule A one-fifth (20%) of the amount for which you made the election. In column E of Part 1 (Schedule A), list the date of the gift as the calendar year for which you are deemed to have made the gift (that is, the year of the current Form 709 you are filing). Do not list the actual year of contribution for subsequent years.

However, if in any of the last 4 years of the election, you did not make any other gifts that would require you to file a Form 709, you do not need to file Form 709 to report that year's portion of the election amount."