new Date("…") gives Invalid Date or the wrong day on V8

Symptoms after migrating: Invalid Date in logs, NaN in sheets, or every date showing one day early.

Why it happens

Only the ISO format is standardised. Everything else ("12/31/2025", "Dec 31 2025 10:00", "31.12.2025") is engine-specific, so Rhino and V8 disagree on some strings. And a date-only ISO string is read as UTC midnight:

new Date('2026-01-05');          // 2026-01-05T00:00Z → shows Jan 4, 7pm in New York
new Date('2026-01-05T00:00:00'); // local midnight

The fix: build dates from parts

// Fragile
var cutoff = new Date('12/31/2025');

// Explicit (local time, month is 0-based)
var cutoff = new Date(2025, 11, 31);

// Parse a known format from a sheet or API
function parseYmd(s) {
  var p = s.split('-');
  return new Date(+p[0], +p[1] - 1, +p[2]);
}

Dates read with Range.getValues() from real date cells are already Date objects — don't round-trip them through strings.

How V8Shift handles it

Literal M/D/YYYY strings are rewritten to new Date(y, m - 1, d) (same meaning, explicit). Other literal date strings and Date.parse() calls are flagged with advice.