Clock letters that are never all lit at the same time

• Day-and-night, in a way
• Clock radio switch
• Letters about time
• Clock-setting button, and the theme of the puzzle
• Convenience store chain
• Convenience store connected to BP
• Switch on a clock radio
• Time-setting choice
• Around-the-clock, in a way

Wikipedia Word definitions in Wikipedia
thumb|right|upright=1.0|A BP with an ampm in Cranberry Township, Pennsylvania near Pittsburgh right off the Pennsylvania Turnpike . The ampm brand (rarely "am/pm") is a convenience store chain with branches located in several U.S. states, including Arizona ...