Bugfix: datetime parsing and local timezones (#15544)

Hi,
This PR should close 3 issues
- [DMY date format is parsed inconsistently
#14123](https://github.com/nushell/nushell/issues/14123)
- [into datetime doesnt't work with --format and ignores user's locale
#11015](https://github.com/nushell/nushell/issues/11015)
- [into datetime: iinconsistent and incrrect behaviour regarding
timezones #13823](https://github.com/nushell/nushell/issues/13823)


# Description
- Allow to parse only dates or only times with --format
- Use local timezone depending on the input. Ex: I'm in France, so show
dates with +0100 in winter and +0200 in summer.

```nushell
# Concerning #13823

> "2020-01-01 12:00" | into datetime
Wed, 1 Jan 2020 12:00:00 +0100 (5 years ago)
# OK, it's my timezone in winter time

> "2020-06-01 12:00" | into datetime
Mon, 1 Jun 2020 12:00:00 +0200 (4 years ago)
# OK, it's my timezone in summertime

> ("2024-10-27 12:00" | into datetime) - ("2024-10-27 00:00" | into datetime)
13hr
# Ok, because we switched from summer to winter time on 2025-10-27, so there are actually 13h between midnight and noon

> "2020-01-01 12:00" | into datetime --format "%Y-%m-%d %H:%M"
Wed, 1 Jan 2020 12:00:00 +0100 (5 years ago)
# OK: timezone is assumed to be local, and +0100 is my timezone in winter

# Concerning #14123 and #11015
# Flexible parsing still works like before, which could be counter-intuitive, but it's flexible parsing
# with one difference: the timezone is local
> '12-01-2001' | into datetime
Sat, 1 Dec 2001 00:00:00 +0100 (23 years ago)
# OK, +0100 is my timezone in winter time. If I run it with nushell 0.103.0 in summer time, I get +0200
> '13-01-2001' | into datetime
Sat, 13 Jan 2001 00:00:00 +0100 (24 years ago)

## If you want, you can use the --format option to parse a date or a time (before, it had to be a date + time)
## Notice here again the timezone is correct depending on winter/summer time
~> "06.03.2023" | into datetime -f "%d.%m.%Y"
Mon, 6 Mar 2023 00:00:00 +0100 (2 years ago)
~> "06.03.2023" | into datetime -f "%m.%d.%Y"
Sat, 3 Jun 2023 00:00:00 +0200 (2 years ago)
> "10:00" | into datetime --format "%H:%M"
Thu, 10 Apr 2025 10:00:00 +0200 (9 hours ago)
```

# User-Facing Changes
See above

# Tests + Formatting


# After Submitting
I'll down something for the release notes, if this is merged in time 😄

crates/nu-command/src/conversions/into/datetime.rs

@@ -458,13 +458,8 @@ fn action(input: &Value, args: &Arguments, head: Span) -> Value {
                     }
                 },
                 Err(reason) => {
-                    match NaiveDateTime::parse_from_str(val, &dt_format.item.0) {
+                    match parse_with_format(val, &dt_format.item.0, head) {
-                        Ok(d) => {
+                        Ok(parsed) => parsed,
-                            let dt_fixed =
-                                Local.from_local_datetime(&d).single().unwrap_or_default();
-
-                            Value::date(dt_fixed.into(),head)
-                        }
                         Err(_) => {
                             Value::error (
                                 ShellError::CantConvert { to_type: format!("could not parse as datetime using format '{}'", dt_format.item.0), from_type: reason.to_string(), span: head, help: Some("you can use `into datetime` without a format string to enable flexible parsing".to_string()) },
@@ -808,6 +803,34 @@ fn parse_timezone_from_record(
     }
 }
 
+fn parse_with_format(val: &str, fmt: &str, head: Span) -> Result<Value, ()> {
+    // try parsing at date + time
+    if let Ok(dt) = NaiveDateTime::parse_from_str(val, fmt) {
+        let dt_native = Local.from_local_datetime(&dt).single().unwrap_or_default();
+        return Ok(Value::date(dt_native.into(), head));
+    }
+
+    // try parsing at date only
+    if let Ok(date) = NaiveDate::parse_from_str(val, fmt) {
+        if let Some(dt) = date.and_hms_opt(0, 0, 0) {
+            let dt_native = Local.from_local_datetime(&dt).single().unwrap_or_default();
+            return Ok(Value::date(dt_native.into(), head));
+        }
+    }
+
+    // try parsing at time only
+    if let Ok(time) = NaiveTime::parse_from_str(val, fmt) {
+        let now = Local::now().naive_local().date();
+        let dt_native = Local
+            .from_local_datetime(&now.and_time(time))
+            .single()
+            .unwrap_or_default();
+        return Ok(Value::date(dt_native.into(), head));
+    }
+
+    Err(())
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

crates/nu-command/src/date/utils.rs

@@ -9,7 +9,11 @@ pub(crate) fn parse_date_from_string(
         Ok((native_dt, fixed_offset)) => {
             let offset = match fixed_offset {
                 Some(offset) => offset,
-                None => *(Local::now().offset()),
+                None => *Local
+                    .from_local_datetime(&native_dt)
+                    .single()
+                    .unwrap_or_default()
+                    .offset(),
             };
             match offset.from_local_datetime(&native_dt) {
                 LocalResult::Single(d) => Ok(d),