import pandas as pd import numpy as np dt = np.datetime64("2016-01-01", "ms") ts = pd.Timestamp(dt) >>> ts.value 1451606400000 # <- 2.0/main 1451606400000000000 # <- 1.x

In previous versions .value has always been in nanoseconds. By changing the reso we get with some Timestamp/Timedelta inputs, we change the .value, which is technically public API.

One option would be just to document the change in .value behavior along with the other breaking changes docs.

Another would be to use e.g. ._value internally and keep .value as representing nanos.