And where did you read that?  Maybe some AI-created summary?

"§ 1.1377-1 Pro rata share.

(a) Computation of pro rata shares—(1) In general. For purposes of subchapter S of chapter 1 of the Internal Revenue Code and this section, each shareholder's pro rata share of any S corporation item described in section 1366(a) for any taxable year is the sum of the amounts determined with respect to the shareholder by assigning an equal portion of the item to each day of the S corporation's taxable year, and then dividing that portion pro rata among the shares outstanding on that day. See paragraph (b) of this section for rules pertaining to the computation of each shareholder's pro rata share when an election is made under section 1377(a)(2) to treat the taxable year of an S corporation as if it consisted of two taxable years in the case of a termination of a shareholder's entire interest in the corporation."