Below zero does not count as chilling hours. The temperature has to be higher than freezing and lower than 60F.
Notice in the following model that tempertures over 60F degrees result in negative chill units (you lose chilling units), and none accumulate under 34F degrees.
1 hour below 34 degrees F =0.0 chill unit
1 hour 35-36 degrees F =0.5 chill units
1 hour 37-48 degrees F =1.0 chill units
1 hour 49-54 degrees F =0.5 chill units
1 hour 55-60 degrees F =0.0 chill units
1 hour 61-65 degrees F =-0.5 chill units
1 hour >65 degrees F =-1.0 chill units
Hope this helps to calculate chilling hours.
Hardiness Zone 6a
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