Data Averaging every 30 seconds for uneven time intervals

To all the excel wizards out there,
I need some help averaging heart rate data every thirty seconds. I believe there was an earlier thread regarding a similar issue but the solution does not seem to work for my data.
Basically the data that I have is data taken from a device that measures heart rate for every beat (Ideally, assuming the device is correctly installed on the patient). Now I want to find the average heart rate for every thirty seconds that have passed during a monitoring period.
I have attached my excel file to give you all a better understand of the numbers I have to play with. The time elapsed column is when a heart beat was detected and the time (in seconds) it was recorded, which starts from when the device was turned on (for example if a heart rate was detected at 882 seconds after the device was turned on, then that one heart beat would be recorded as taking place at 882 seconds). Even though the device was left on for half an hour there was only a 4 minute window of actual testing this is why the first number in the coulmn is 875.202 (seconds). I have only provided a one minute sample of the data that was actually pulled from the device.
The second coulmn is the acutal time (during the day) that the heart beat was detected. The third coulmn is the value of the heart beat.

As I stated before I wish to average the data for every thirty seconds that passed during the monitoring period (in some of the data there may be a 3-5 second gap in heart beat detection, this is due to the device not being properly placed). This may seem like a simple solution; simply count for every 30 seconds and average the data in between. Problem is their are 4 test subjects in total and each patient has 6 monitoring periods, all 4-5 minutes in length. I would be sitting at my desk for awhile.