Being complete and whole in the moment without crutch or dependency to the past or the future, either recent or distant, has been a goal of several spiritual traditions, perhaps Zen most noticeably. The western popularizer of that tradition, Alan Watts, certainly established it in his many books of the 60’s and 70’s, several of which came my way back then. I also recall Ram Dass (aka Richard Alpert) spreading the notion with his popular book Be Here Now in that same decade.
Mindfulness is the practice of purposely focusing your attention on the present moment and accepting it without judgment. Mindfulness is now being examined scientifically and has been found to be a key element in stress reduction and overall happiness. The past is already gone, the future has not yet come, you only have the present, so be mindful of it