Significant events for Maryland overlays
| Date |
Details |
| Maryland has had several overlays. This is the detail. | |
| WESTERN | |
| October 6, 1991 | Area code 240 was overlaid on the western area of Maryland. |
| June 14, 2023 | Area code 227 was overlaid on the western area of Maryland. |
| EASTERN | |
| July 1, 1997 | Area code 443 was overlaid on the eastern area of Maryland. |
| August 24, 2012 | Area code 667 was overlaid on the eastern area of Maryland. |