def cert_get_not_before(cert):
    return cert.not_valid_before
Return the Categorical without null values.
dataset/ETHPy150Open Netflix/security_monkey/security_monkey/watchers/iam/iam_ssl.py/cert_get_not_before--crossover with--dataset/ETHPy150Open pydata/pandas/pandas/core/categorical.py/Categorical.dropna