def appendFieldAfter(fc,datetimeStart):
    afterRows = arcpy.da.SearchCursor(fc, ["OID@", "datetimestart"])
    for row in afterRows:
        before = datetimeStart[row[0]]
        after = row[1]
        datetimeStart[row[0]] = [before,after]
    return datetimeStart
add the date/time field values after the change
dataset/ETHPy150Open Esri/solutions-geoprocessing-toolbox/data_management/test/test_adjust_sample_dates/TestAdjustSampleDataDates.py/appendFieldAfter