The characters literally combine to mean “Today is not comparable to the past.” The idiom encapsulates the idea that the gap between the present and the past is so vast that a simple comparison is no longer meaningful—the present is just on a completely different level.
今非昔比 is deeply rooted in a cultural awareness of history and progress. In modern China, it's more than just an idiom; it's a reflection of the national narrative of rapid development and rejuvenation over the past several decades. It is frequently used with a sense of collective pride to describe the country's economic miracles, technological leaps, and improved living standards since the “Reform and Opening Up” period. A Westerner might say “it's a night and day difference” or “it's a world of difference.” While similar, these phrases focus more on the magnitude of the difference itself. 今非昔比 carries a stronger temporal element—it specifically contrasts the present with the past, implying a journey of progress. This aligns with the cultural value of remembering past hardships (吃苦, chīkǔ) to better appreciate present prosperity. It's an optimistic idiom that celebrates growth and transformation.
This idiom is extremely common and can be used in both formal and informal settings. Its connotation is overwhelmingly positive.