Precise age scales are essential for reconstructing past climate variability at high temporal resolution. Here, we present a new oxygen-isotope record for the SE-Dome II ice core from southeast Greenland and construct a multi-month-resolution age scale spanning 1871–2020 CE. The chronology, named SE2025iso, was developed by aligning oxygen-isotope variations in the ice core with isotope-enabled climate-model simulations. High snow accumulation at the SE-Dome site preserved multi-month variations of oxygen isotopes, allowing age determination beyond seasonal extremes. Although absolute validation at the monthly scale is challenging, the chronology has an estimated mean uncertainty of 2.0 months and is supported by multiple consistency evaluations, including melt and tritium horizons, seasonal accumulation comparisons, and also volcanic reference layers. The reconstructed snow accumulation rates correlate well with climate reanalysis data, except in summer. The reconstructed accumulation record shows that the weak seasonality at SE-Dome persisted throughout the past 150 years. This persistence suggests that the balance near the boundary between summer- and winter-dominated accumulation regimes across Greenland remained broadly stable despite recent Arctic warming. Applying the SE2025iso age scale to chemical records shows that hydrogen peroxide concentrations peak approximately 2–3 weeks after the summer solstice and that volcanic sulfate records exhibit reproducible post-eruption peaks at multi-month resolution. Overall, the SE2025iso chronology highlights the potential of high-accumulation ice-core sites for investigating multi-month-scale climate variability and aerosol deposition processes.