[ARROW-10213] [C++] Temporal cast from timestamp to date rounds instead of extracting date component

• Type: Bug
• Status: Closed
• Priority: Minor
• Resolution: Duplicate
• Affects Version/s: 1.0.1
• Fix Version/s: 6.0.0
• Component/s: C++
• External issue URL: https://github.com/apache/arrow/issues/26216
• Assignee: David Li
• Reporter: David Li
• Created: 07/Oct/20 12:56
• Updated: 11/Jan/23 08:11
• Resolved: 09/Sep/21 13:54

Description

I'd expect this code to give 1950-01-01 twice (i.e. a timestamp -> date cast extracts the date component, ignoring the time component):

import datetime
import pyarrow as pa
arr = pa.array([
    datetime.datetime(1950, 1, 1, 0, 0, 0),
    datetime.datetime(1950, 1, 1, 12, 0, 0),
], type=pa.timestamp("ns"))
print(arr)
print(arr.cast(pa.date32(), safe=False)) 

However it gives 1950-01-02 in the second case:

[
  1950-01-01 00:00:00.000000000,
  1950-01-01 12:00:00.000000000
]
[
  1950-01-01,
  1950-01-02
]

The reason is that the temporal cast simply divides, and C truncates towards 0 (note: Python truncates towards -Infinity, so it would give the right answer in this case!), resulting in -7304 days instead of -7305.

Depending on the intended semantics of a temporal cast, either it should be fixed to extract the date component, or the rounding behavior should be noted and a separate kernel should be implemented for extracting the date component.

Issue Links

• duplicates: ARROW-13549 [C++] Implement timestamp to date/time cast that extracts value