What is before spring? Genjokoan says

Birth is an expression complete this moment. Death is an expression
complete this moment. They are like winter and spring. You do not call
winter the beginning of spring, nor summer the end of spring.

Dogen says here that spring is not after winter, but there is still &quot;before&quot; and &quot;after&quot;, which is included in spring, and spring is independent of them. etc.

If not the beginning of spring, what is before spring? Not winter.

And if - likewise - the end of life is discontinuous with the present, does that mean life does not end when I only exist in the present?
This idea is elaborated in Dogen's work called &quot;Uji&quot; (The Time-Being).
Here I will paste some bits and pieces:

Time itself is being, and all being is time.
Each moment is all being, is the entire world. Reflect now whether any being or any world is left out of the present moment.
In essence, all things in the entire world are linked with one another as moments. Because all moments are the time-being, they are your time-being.
Time is not separate from you, and as you are present, time does not go away.
Do not think that time merely flies away. Do not see flying away as the only function of time. If time merely flies away, you would be separated from time.
The time-being has the quality of flowing. So-called today flows into tomorrow.
You may suppose that time is only passing away, and not understand that time never arrives.
People only see time's coming and going, and do not thoroughly
understand that the time-being abides in each moment.
Do not think flowing is like wind and rain moving from east to west. The entire world is not unchangeable, is not immovable. It flows. Flowing is like spring. Spring with all its numerous aspects is called flowing. When spring flows there is nothing outside of spring. Study this in detail. Spring invariably flows through spring. Although flowing itself is not spring, flowing occurs throughout spring. Thus, flowing is completed at just this moment of spring. Examine this thoroughly, coming and going. In your study of flowing, if you imagine the objective to be outside yourself and that you flow and move through hundreds and thousands of worlds, for hundreds, thousands, and myriads of eons, you have not devotedly studied the buddha way.

What does this all mean? It means there is no hidden essence or identity that  would pass from the past into the future. When milk goes sour and &quot;turns into&quot; yogurt, or firewood burns out and &quot;turns into&quot; ash, or spring ends and summer starts, or &quot;a person dies&quot; - we habitually assume that something, some entity, stays itself throughout all phases and gets transferred from point A to point B to point C. However in reality there is no such thing. Identity is our imputation.
The young person, the old person, and the corpse are not the same entity - they are like the dominos falling in succession, like a moving wave. Similarly, the spring and summer are not the same entity, they are phases of flowing like the dominos in a chain.
Entire world is flowing like that, and most importantly the illusion that &quot;I am staying the same as the time flows&quot; or &quot;I am moving through time while staying myself&quot; - is the illusion of self. Our mind and our experience is part of the flow.
This is why, in Genjokoan:

It is a custom in Buddhism not to say that life turns into death. This is why we call it “unborn”. And it is an established Buddhist teaching since Gautama Buddha that death does not turn into life. This is why we call it “undying”. Life is an instantaneous situation, and death is also an instantaneous situation.