You can do this with a trigger.
Here's one possible way, although there are many:
1. When you have the guy in the jeep tell you about the tanks, in the waypoint located next to you, put the following code in the Activation field:
StartTimer = true
2. Create an init.sqs script if you don't already have one, and put in the following code:
StartTimer = false
3. Create a trigger with the low, mid, and high times set to 600 (10 minutes), select the Countdown option, and change the Condition field to the following code:
StartTimer
4. In that trigger's On Activation field, make whatever happen that you want to happen.
So, when the StartTimer global variable is set to true, the 10-minute countdown will begin at that time.