test.py

correct_precip = []
for t in df.DateTime:
    if t.hour not in [0,6,12,18]:
        precip_1 = df.precip[df.DateTime==t]-df.precip[df.DateTime==t-relativedelta(hours=1)]
        dat = {'DateTime' : t, 'precipitation' : precip_1}
        correct_precip.append(dat)
    else:
        dat = {'DateTime' : t, 'precipitation' : df.precip[df.DateTime==t]}
        correct_precip.append(dat)