Precise age constraints for the onset and duration of the Paleocene–Eocene Thermal Maximum (PETM) areessential for understanding the mechanisms that triggered and sustained this major climate perturbation.However, establishing a precise PETM geochronology is complicated by uncertainties in orbital tuning and acurrent lack of precise radiometrically dated marker horizons. An early Eocene rhyolitic ash layer named +19 is apromising marker horizon due to its distinct geochemistry, its occurrence in key offshore archives, and wellpreservedoutcrops in Denmark such as Stolleklint, where it conformably overlies PETM strata. We present anew high-precision U-Pb zircon age of 55.331 ± 0.053/(0.060)/[0.080] Ma for ash +19, supported by a new Ar-Ar age of 55.424 ± 0.115/(0.116)/[0.320] Ma within uncertainty. The ash +19 U-Pb age enables direct comparisonwith ash SB01–1 in Svalbard that was erupted during the PETM carbon isotope excursion (CIE), yieldinga 454 ± 90 kyr interval between the two layers. This provides a robust geochronological link from the PETM toash +19 that is independent of stratigraphic interpretations. Using end-member PETM durations (94–170 kyr),we estimate the time between the PETM onset and ash +19 as 528–604 ± 102 kyr, which is significantly shorterthan the 862 ± 20 kyr interval derived from astronomical tuning. Adopting the longer helium isotope-basedPETM duration yields an onset age of 55.935 ± 0.102 Ma, with a CIE recovery end at 55.700 Ma. A shorterPETM duration requires a younger PETM onset age. The positive ash series in Denmark, correlating with theoffshore Balder Formation, is now constrained to a 250 kyr interval between 55.367 and 55.117 ± 0.080 Ma.This refined age model provides a robust framework for testing and improving early Eocene astronomical timescales.