noun
unchanging for years; same old routine for ages
Describes a state or activity that continues without change for a very long time, often with a nuance of monotony or steadfastness. Relatively rare and literary.
彼の生活は千年一日のごとく変わらなかった。
His life remained unchanged, as if a thousand years were a single day.
十年一日 means 'unchanging for ten years', a similar but less extreme expression than 千年一日.
A four-character compound from 千年 (a thousand years) and 一日 (one day), literally 'a thousand years as one day', expressing an unchanging state over a very long period.