A seven-day delay cap and annual changes below 0.76 day permit zero lilac interval slopes in four western US samples, 1967-2008

We tested whether slow changes in separate leaf and flower reporting delays can remove fitted warmth slopes of the leaf-to-bloom gap. We used 3,004 site-year records for common lilac in two western US regions and two periods from 1967 to 2008. Models removed site and year effects and winter temperature. Recorded April-May slopes ranged from -2.22 to -0.36 days per C. With each phase delay between zero and seven days, annual change limits of 0.0865 to 0.7502 day permit a zero slope in all four samples. Corrected bloom must occur on or after corrected leaf. Equal delays for both phases cancel. These are exact sample sensitivity bounds under stated assumptions, not confidence intervals or evidence of actual delays. Unknown visits, selected long records, coarse monthly weather and overlapping source observations limit interpretation. The package includes data, hashes, code, bounds, explicit delay values and numerical checks. No pollution or causal climate effect was tested.

phenology · observation bias · lilac · reporting delay · linear programming · ecological timing