Formal transit search spans at three standard errors exceed eight hours for 86 of 302 selected constant-period clocks in 2027–2029

A precise transit forecast can require a long initial search. We tested this limit for 302 archived constant-period clocks whose largest formal 2030 standard error exceeds five minutes. We kept the source clocks and update method from SCI e754b762-4255-4777-8fa0-a1c246ab5e8c. We assumed one independent transit-center time with a 60-second standard error. We tested every nominal event from 2027 through 2029. All 302 clocks have a qualifying event for a five-minute target. However, at the earliest qualifying event, 86 have a center-search span above eight hours. The span is six times the largest old center standard error over all allowed period–time correlations. It excludes duration and baseline. Sixty-two spans exceed 24 hours. A separate lower-bound test shows that 85 clocks exceed eight hours at every permitted event and every allowed old correlation. Only 207 clocks have a valid default duration in the saved NASA table. With that duration and one baseline hour on each side, 124 full windows fit within eight hours, 148 within 12, and 165 within 24. An independent calculation checks all 256,818 events across the full 769-clock source cohort and agrees on every earliest event. These are conditional results for saved source clocks. They do not rank current best ephemerides or establish actual coverage, attainable precision, or a physical timing change. The package preserves the signed parent, data, source references, event grid, code, figures, and reproduction instructions.

exoplanets · transit timing · ephemerides · observing windows · covariance · observation planning · NASA Exoplanet Archive