The source[1] says significant construction begins in spring 2027 (no day; the first day of spring is used) and continues through 2033 (year only; the last day is used).
The Army Corps of Engineers awarded Barnard Construction a $1.13 billion contract for the Prado Dam Spillway Modification Project: a labyrinth-style spillway control structure, rebuilt chute walls and slabs, and a 20-foot higher spillway crest. Significant construction is expected to begin in spring 2027 and continue through 2033.
$1.1B