to keep

"to obey"

that your days may be prolonged

Long days are a metaphor for a long life. This can be stated in active form. This can be translated similarly to "prolong your days" in Deuteronomy 4:26. Alternate translation: "that I may prolong your days" or "that I may cause you to live a long time" (See: rc://en/ta/man/translate/figs-metaphor and rc://en/ta/man/translate/figs-activepassive)