divisors multiples – How accurate is my calculation of 1/3 of a time duration?

Suppose:

1. I have time of the day X in HH:mm
2. Time of the day Y (always later than time X) in HH:mm
3. I calculate their difference and store it in milliseconds
4. Divide the difference by 3
5. Subtract that very same 1/3 of the difference in milliseconds from time Y

I can then display when the 3rd part of the time duration between X and Y begins in either HH:mm:ss or HH:mm.

Either way, I want to state a disclaimer saying “this is accurate to the nearest …” or “This is the result, give or take …”

If I display it in HH:mm, how accurate would it be? To the nearest minute? Should I say “This is when the 3rd part of the time duration between X and Y begins, give or take 1 minute”?

If I display it in HH:mm:ss, how accurate would it be? To the nearest second? Should I say “This is when the 3rd part of the time duration between X and Y begins, give or take 1 second”?