Add unix epoch as timezone option (#5450)

* Closes #4866

### What
Added a unix epoch timestamp display option, as mentioned in issue
[4866](#4866)

<img width="1470" alt="Screenshot 2024-03-10 at 23 25 16-min"
src="https://github.com/rerun-io/rerun/assets/21971561/c18ae1c1-9eb5-494a-bf30-78d2e0f7f363">

Not quite sure how compact time display should be handled: 

```diff
    /// Useful when showing dates/times on a timeline
    /// and you want it compact.
    ///
    /// Shows dates when zoomed out, shows times when zoomed in,
    /// shows relative millisecond when really zoomed in.
    pub fn format_time_compact(&self, time_zone_for_timestamps: TimeZone) -> String {
...
            if let Some(datetime) = self.to_datetime() {
                let is_whole_minute = ns % 60_000_000_000 == 0;
                let time_format = if self.is_exactly_midnight() {
                    "[year]-[month]-[day]"
                } else if is_whole_minute {
                    "[hour]:[minute]"
                } else {
-                    "[hour]:[minute]:[second]"
+                    Self::get_time_prefix(&time_zone_for_timestamps)
                };
                let parsed_format = time::format_description::parse(time_format).unwrap();

                return Self::time_string(datetime, &parsed_format, time_zone_for_timestamps);
            }
...
}
``` 

Should the `[hour]:[minute]` be changed as well when unix timestamp is
chosen?

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using newly built examples:
[app.rerun.io](https://app.rerun.io/pr/5450/index.html)
* Using examples from latest `main` build:
[app.rerun.io](https://app.rerun.io/pr/5450/index.html?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[app.rerun.io](https://app.rerun.io/pr/5450/index.html?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!

- [PR Build Summary](https://build.rerun.io/pr/5450)
- [Docs
preview](https://rerun.io/preview/7e0f542930b6d1279c55190e3f375f0dcf2b2603/docs)
<!--DOCS-PREVIEW-->
- [Examples
preview](https://rerun.io/preview/7e0f542930b6d1279c55190e3f375f0dcf2b2603/examples)
<!--EXAMPLES-PREVIEW-->
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

---------

Co-authored-by: mm <murgeljm@gmail.com>
Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/re_log_types/src/time.rs

@@ -6,6 +6,9 @@ use time::{format_description::FormatItem, OffsetDateTime, UtcOffset};
 pub enum TimeZone {
     Local,
     Utc,
+
+    /// Seconds since unix epoch
+    UnixEpoch,
 }
 
 /// A date-time represented as nanoseconds since unix epoch
@@ -91,6 +94,7 @@ impl Time {
             TimeZone::Utc => {
                 format!("{}Z", datetime.format(&parsed_format).unwrap())
             }
+            TimeZone::UnixEpoch => datetime.format(&parsed_format).unwrap(),
         }
     }
 
@@ -101,19 +105,23 @@ impl Time {
         if let Some(datetime) = self.to_datetime() {
             let is_whole_second = nanos_since_epoch % 1_000_000_000 == 0;
             let is_whole_millisecond = nanos_since_epoch % 1_000_000 == 0;
+            let prefix = match time_zone_for_timestamps {
+                TimeZone::UnixEpoch => "[unix_timestamp]",
+                TimeZone::Utc | TimeZone::Local => "[hour]:[minute]:[second]",
+            };
 
             let time_format = if is_whole_second {
-                "[hour]:[minute]:[second]"
+                prefix.to_owned()
             } else if is_whole_millisecond {
-                "[hour]:[minute]:[second].[subsecond digits:3]"
+                format!("{prefix}.[subsecond digits:3]")
             } else {
-                "[hour]:[minute]:[second].[subsecond digits:6]"
+                format!("{prefix}.[subsecond digits:6]")
             };
 
             let date_is_today = datetime.date() == OffsetDateTime::now_utc().date();
             let date_format = format!("[year]-[month]-[day] {time_format}");
             let parsed_format = if date_is_today {
-                time::format_description::parse(time_format).unwrap()
+                time::format_description::parse(&time_format).unwrap()
             } else {
                 time::format_description::parse(&date_format).unwrap()
             };
@@ -147,9 +155,15 @@ impl Time {
                 let time_format = if self.is_exactly_midnight() {
                     "[year]-[month]-[day]"
                 } else if is_whole_minute {
-                    "[hour]:[minute]"
+                    match time_zone_for_timestamps {
+                        TimeZone::UnixEpoch => "[unix_timestamp]",
+                        TimeZone::Utc | TimeZone::Local => "[hour]:[minute]",
+                    }
                 } else {
-                    "[hour]:[minute]:[second]"
+                    match time_zone_for_timestamps {
+                        TimeZone::UnixEpoch => "[unix_timestamp]",
+                        TimeZone::Utc | TimeZone::Local => "[hour]:[minute]:[second]",
+                    }
                 };
                 let parsed_format = time::format_description::parse(time_format).unwrap();
 
@@ -302,6 +316,19 @@ mod tests {
             &Time::from_us_since_epoch(69_900).format(TimeZone::Local),
             "+0.070s"
         );
+
+        assert_eq!(
+            &Time::from_us_since_epoch(42_000_000).format(TimeZone::UnixEpoch),
+            "+42s"
+        );
+        assert_eq!(
+            &Time::from_us_since_epoch(69_000).format(TimeZone::UnixEpoch),
+            "+0.069s"
+        );
+        assert_eq!(
+            &Time::from_us_since_epoch(69_900).format(TimeZone::UnixEpoch),
+            "+0.070s"
+        );
     }
 
     #[test]

crates/re_viewer/src/ui/rerun_menu.rs

@@ -319,12 +319,22 @@ fn options_menu_ui(
 
     ui.horizontal(|ui| {
         ui.label("Timezone:");
+    });
+    ui.horizontal(|ui| {
         re_ui
             .radio_value(ui, &mut app_options.time_zone, TimeZone::Utc, "UTC")
             .on_hover_text("Display timestamps in UTC");
         re_ui
             .radio_value(ui, &mut app_options.time_zone, TimeZone::Local, "Local")
             .on_hover_text("Display timestamps in the local timezone");
+        re_ui
+            .radio_value(
+                ui,
+                &mut app_options.time_zone,
+                TimeZone::UnixEpoch,
+                "Unix Epoch",
+            )
+            .on_hover_text("Display timestamps in seconds since unix epoch");
     });
 
     {