Calculating a rolling 30-day valid-test average (without zeros)

Need help with calculating a rolling 30-day valid-test average. Tests with a result of zero are deemed invalid for this discussion. The zeros must be excluded from the average, which must consist of 30 valid results.

The challenge here is the variable number of values which get evaluated... for example, 2/6/2010 in the attached example requires 36 calendar days to obtain 30 valid tests for the average. The frequency of zeros is also random. Help!

Thanks in advance,

Air Hugger