Use local timezone, not local UTC offset

With this change, we treat "local" as the local timezone, whereas
before, we treated "local" as the local (current) UTC offset.

The following behavior is shown for timezone Europe/Berlin, during
winter.

Before: UTC offset is used for parse_datetime

    >>> parse_datetime("2024-01-01T12:00:00Z")
        = Mon, 1 Jan 2024 12:00:00 +0000    [DateTime]

After:

    >>> parse_datetime("2024-01-01T12:00:00Z")
        = Mon, 1 Jan 2024 13:00:00 +0100    [DateTime]

Before: "Winter" UTC offset is used for datetime in summer

    >>> parse_datetime("2024-07-01T12:00:00Z") -> "local"
        = Mon, 1 Jul 2024 13:00:00 +0100    [DateTime]

After: "Summer" UTC offset is used for datetime in summer

    >>> parse_datetime("2024-07-01T12:00:00Z") -> "local"
        = Mon, 1 Jul 2024 14:00:00 +0200    [DateTime]

After: "Winter" UTC offset is used for datetime in winter

    >>> parse_datetime("2024-01-01T12:00:00Z") -> "local"
        = Mon, 1 Jan 2024 13:00:00 +0100    [DateTime]

Author: sharkdp

Cargo.lock

@@ -31,6 +31,7 @@ num-format = "0.4.4"
 walkdir = "2"
 chrono = "0.4.31"
 chrono-tz = "0.8.5"
+iana-time-zone = "0.1"
 
 [features]
 default = ["fetch-exchangerates"]

numbat/Cargo.toml

@@ -0,0 +1,22 @@
+use chrono::{DateTime, FixedOffset, Offset, TimeZone};
+use chrono_tz::Tz;
+
+fn get_local_timezone() -> Option<Tz> {
+    let tz_str = iana_time_zone::get_timezone().ok()?;
+    tz_str.parse().ok()
+}
+
+/// We use this function to get the UTC offset corresponding to
+/// the local timezone *at the specific instant in time* specified
+/// by 'dt', which might be different from the *current* UTC offset.
+///
+/// For example, in the timezone Europe/Berlin in 2024, we expect a
+/// UTC offset of +01:00 for 'dt's in winter, while we expect a UTC
+/// offset of +02:00 for 'dt's in summer, due to DST. If we were to
+/// use chronos 'Local', we would get a UTC offset depending on when
+/// we run the program.
+pub fn local_offset_for_datetime<O: TimeZone + Offset>(dt: &DateTime<O>) -> FixedOffset {
+    get_local_timezone()
+        .map(|tz| dt.with_timezone(&tz).offset().fix())
+        .unwrap_or_else(|| dt.offset().fix())
+}

numbat/src/datetime.rs

@@ -805,9 +805,9 @@ fn parse_datetime(args: &[Value]) -> Result<Value> {
         .or_else(|_| chrono::DateTime::parse_from_rfc2822(input))
         .map_err(RuntimeError::DateParsingError)?;
 
-    let offset = output.offset();
+    let offset = crate::datetime::local_offset_for_datetime(&output);
 
-    Ok(Value::DateTime(output.into(), *offset))
+    Ok(Value::DateTime(output.into(), offset))
 }
 
 fn format_datetime(args: &[Value]) -> Result<Value> {

numbat/src/ffi.rs

@@ -3,6 +3,7 @@ mod ast;
 mod bytecode_interpreter;
 mod column_formatter;
 mod currency;
+mod datetime;
 mod decorator;
 pub mod diagnostic;
 mod dimension;

numbat/src/lib.rs

@@ -697,7 +697,7 @@ impl Vm {
                     let lhs = self.pop_datetime();
 
                     let offset = if rhs == "local" {
-                        chrono::Local::now().offset().fix()
+                        crate::datetime::local_offset_for_datetime(&lhs)
                     } else {
                         let tz: chrono_tz::Tz = rhs
                             .parse()