It never shows itself. It neither warns nor gives you time to get ready. It just happens, quietly, and then it's gone before you fully comprehend what it meant.
Sometimes it's a little thing. Sitting in a typical place, hearing a giggle that lingers longer than anticipated. Watching someone walk away and understanding without stating it aloud, that things will never be the same. Nothing significant, nothing forceful, just a change you sense more than you can put into words.
In some times, you could make things wait. Not because they are perfect, but because you can see that they are sliding even while you are still inside them. You work to retain them, to stretch them simply longer, but time passes for no one. It continues moving, steady and uninterested.
Then there are instances you only later recognize. They appear unremarkable, nearly unmemorable at the time.