Q: I saw the following sentence in the New York Times: “But it works because the critical mass of viewers gathers before TVs in prime time.” Is it proper English? I’m thinking it should be “at prime time.”
A: We usually use “at” when referring to a specific time (“at 3 o’clock” or “at 7 PM”) and “in” or “during” when referring to a more general period of time (“in the afternoon” or “during the evening”).
The American Heritage Dictionary of the English Language (4th ed.) defines “prime time” this way: “The evening hours, generally between 7 and 11 P.M., when the largest television audience is available.”
Since “prime time” refers to a period of time, not a specific hour, an English speaker would generally use “in” or “during” with it. So yes, the author of that Times article was indeed using proper English.
By the way, you may find it interesting that the noun phrase “prime time” is very old, hundreds of years older than television, according to the Oxford English Dictionary.
But when it entered English in the 1400s, it referred to the time of prime, the early-morning canonical hour of prayer. In the 1500s, the term also came to mean springtime and the early period of youth, life, and so on, according to the OED.
The first published reference in the OED to “prime time” used in the broadcasting sense is from a 1947 issue of the Wall Street Journal: “Columbia Broadcasting System, for instance, has an unsold hour of prime time on Tuesday nights, beginning at 9:30.”
By the late 1970s, the phrase was being used in a negative expression (“not ready for prime time”) to mean not yet ready for the task or not quite capable of success.
The latest citation in the OED for this usage, from a 2002 article in Science, refers to cell lines that “are not ready for prime time.”