I had always believed that it should be two words—any time. In fact, I cringed whenever I saw it spelled as one word on No Parking signs, almost exclusively in New York City, because I thought it was wrong. Was I right? Read on!
Today I checked it out and discovered that both spellings are correct. However, the choice of which spelling to use depends on the context. Here are two references:
According to these references, New York City is wrong. It seems that when the phrase is preceded by “at” the term must be spelled as two words. In the context of the sign, there is an implied “at” (NO PARKING AT ANY TIME).
Need I say more?