The elusive meaning of now has been a stumbling block in the development of physics. We understand time dilation from velocity and gravity, even the flipping of time in relativity, yet we've made no progress in explaining the most striking aspects of time: its flow and meaning of now. The basic drawing board of physics known as a space-time diagram ignores these issues, and physicists sometimes perversely treat this absence as a strength and conclude that the flow of time is an illusion. That's backward. As long as the meaning of now eludes us, further advances in understanding time - that key aspect of reality - will continue to be stalled.