Function adjust_to_local_time 

fn adjust_to_local_time<T: ArrowTimestampType>(ts: i64, tz: Tz) -> Result<i64>

This function converts a timestamp with a timezone to a timestamp without a timezone. The display value of the adjusted timestamp remain the same, but the underlying timestamp representation is adjusted according to the relative timezone offset to UTC.

This function uses chrono to handle daylight saving time changes.

For example,

'2019-03-31T01:00:00Z'::timestamp at time zone 'Europe/Brussels'

is displayed as follows in datafusion-cli:

2019-03-31T01:00:00+01:00

and is represented in DataFusion as:

TimestampNanosecond(Some(1_553_990_400_000_000_000), Some("Europe/Brussels"))

To strip off the timezone while keeping the display value the same, we need to adjust the underlying timestamp with the timezone offset value using adjust_to_local_time()

adjust_to_local_time(1_553_990_400_000_000_000, "Europe/Brussels") --> 1_553_994_000_000_000_000

The difference between 1_553_990_400_000_000_000 and 1_553_994_000_000_000_000 is 3600_000_000_000 ns, which corresponds to 1 hour. This matches with the timezone offset for “Europe/Brussels” for this date.

Note that the offset varies with daylight savings time (DST), which makes this tricky! For example, timezone “Europe/Brussels” has a 2-hour offset during DST and a 1-hour offset when DST ends.

Consequently, DataFusion can represent the timestamp in local time (with no offset or timezone information) as

TimestampNanosecond(Some(1_553_994_000_000_000_000), None)

which is displayed as follows in datafusion-cli:

2019-03-31T01:00:00

See test_adjust_to_local_time() for example