Converting days to years in loop while computing values across grid cells

[ThiefOfTime]

Do you have to use loops for this task? If not I would recommend using numpy without loops.
if you have a huge amount of data this way is much much more efficient than loops in combination with numpy.
if we can say, that your data has the dimensions: timexlatitudexlongitude then you can do following:

import numpy as np
# quantity would be given at this point and is an 3-dimensional numpy array.
new_quantity = np.add.reduceat(quantity, range(0, quantity.shape[0], 365))

here is the documentation of reduceat https://docs.scipy.org/doc/numpy/referen...uceat.html
i havent tested it yet, but it may work :)