English Translation
The time will come when you understand this.
Better:
The time will come when you are going to understand this.
The second part is also Future in German.
Furthermore, “Die Zeit , da” is a little bit old-style and grandiose. The whole sentence sounds like being from a 19th century prophecy.
“That will be the day when the light shines on thee.” (Not a translation but to give you an idea of the style.)