Luke says that darkness came over the land from the sixth hour until the ninth hour. These numbers are not modern clock times.
Ancient Jewish and Mediterranean writers commonly numbered the daylight hours from around sunrise, making the sixth hour approximately midday and the ninth about mid-afternoon.
Because daylight length changes by season, ancient numbered hours do not correspond with modern sixty-minute precision throughout the year. As a practical approximation, the sixth hour is around noon and the ninth around 3 p.m.
Luke therefore describes darkness during a substantial portion of the middle and later daylight period. The text itself does not identify a natural mechanism for the darkness, and proposed astronomical explanations go beyond what Luke states.
Converting the hour system helps modern readers visualize the scene without imposing a modern clock on an ancient text. The darkness occurs when daylight would normally be expected.
Luke places this extraordinary darkness immediately around Jesus' death and the tearing of the Temple veil, giving the period strong narrative and theological significance.