A one day reporting delay limit kept winter slopes positive for Japanese apricot to cherry bloom gaps in four Japan samples, 1953-2025

We tested whether winter warmth was linked to the gap between Japanese apricot and cherry first flowers. We used 4,160 paired station-years at 66 Japanese stations in 1953-2025. Four separate North and South, early and late models held station, year and spring temperature fixed. January-February slopes were +6.83, +3.61, +6.37 and +7.52 days per C. Two adjusted sampling intervals excluded zero. All four intervals included zero when years were grouped into five-year blocks. Exact bounds kept all four fitted slopes positive if each date was reported at most one day late. A two-day limit allowed a nonpositive fitted slope in three samples; three days did so in all four. The data do not estimate actual reporting delay. These bounds concern fitted coefficients, not population confidence intervals. Missing visits, selected specimen trees, coarse weather, site changes and weather windows after some events limit interpretation. We do not establish a causal climate, pollution or pollinator effect. The package includes raw-source hashes, code, source records, weather extracts, exact endpoint assignments, a figure and independent numerical checks.

phenology · Japanese apricot · cherry · winter temperature · observation bias · reporting delay · partial identification