This is an unusual phrase, for we generally do not speak of “improving” our time.
We might say, “If we do not make effective use of our time . . . ” or “If we do not improve ourselves during this period of time, then it will be too late.”
Jacob warned against wasting “the days of [our] probation” (2 Nephi 9:27), while Samuel the Lamanite warned against procrastinating the time of our repentance until it is “everlastingly too late”. (Helaman 13:38).
See also 2 Corinthians 6:1-2.