How can I say "there is a possibility, up to 30 mins delay in the time you booked for calling you" shorter?

I have a reservation web form. There is a field named "time". It's a select option like:

# Select a time: 

<select>
    <option>09:00 AM</option>
    <option>10:00 AM</option>
    <option>12:00 PM</option>
    <option>02:00 PM</option>
    <option>04:00 PM</option>
</select>

The user must select one of those times to specify at which time we have to call him/her for finalizing his/her request.

Now, I need to add a short label under that field to mention this possibility:

Probably we call you 30 mins sooner or later than the time you selected

How can I say that sentence much shorter?

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