Without context, I see two options here.
This sentence is either meant to be negative:
"If you are unsure about the time..." (meaning: not sure)
Or it is meant to be positive:
Ensure means: to make certain that (something) will occur or be the case.
If this is the case, the proper expression should be: "If you are sure about the time..." or "If you are certain about the time". Of course, there are even more possibilities.
I hope this helps.