config: Add commit_timestamp(commit) template alias

Adds an extension point for changing which date is displayed in log
formats. The function should return a timestamp, not a formatted string.

CHANGELOG.md

@@ -57,6 +57,9 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 * A new Email template type is added. `Signature.email()` now returns an Email
   template type instead of a String.
 
+* Adds a new template alias `commit_timestamp(commit)` which defaults to the
+  committer date.
+
 ### Fixed bugs
 
 * The `$NO_COLOR` environment variable must now be non-empty to be respected.

cli/src/config/templates.toml

@@ -18,7 +18,7 @@ annotate_commit_summary = '''
 separate(" ",
   change_id.shortest(8),
   pad_end(8, truncate_end(8, author.email().local())),
-  committer.timestamp().local().format('%Y-%m-%d %H:%M:%S'),
+  commit_timestamp(self).local().format('%Y-%m-%d %H:%M:%S'),
 )
 '''
 
@@ -73,7 +73,7 @@ if(root,
       separate(" ",
         format_short_change_id_with_hidden_and_divergent_info(self),
         if(author.email(), author.email().local(), email_placeholder),
-        format_timestamp(committer.timestamp()),
+        format_timestamp(commit_timestamp(self)),
         bookmarks,
         tags,
         working_copies,
@@ -160,6 +160,7 @@ name_placeholder = 'label("name placeholder", "(no name set)")'
 commit_summary_separator = 'label("separator", " | ")'
 
 # Hook points for users to customize the default templates:
+'commit_timestamp(commit)' = 'commit.committer().timestamp()'
 'format_short_id(id)' = 'id.shortest(8)'
 'format_short_change_id(id)' = 'format_short_id(id)'
 'format_short_commit_id(id)' = 'format_short_id(id)'
@@ -256,7 +257,7 @@ if(commit.hidden(),
 separate(" ",
         format_short_change_id_with_hidden_and_divergent_info(commit),
         format_short_signature(commit.author()),
-        format_timestamp(commit.committer().timestamp()),
+        format_timestamp(commit_timestamp(commit)),
         commit.bookmarks(),
         commit.tags(),
         commit.working_copies(),

docs/config.md

@@ -484,6 +484,20 @@ Can be customized by the `format_short_signature()` template alias.
 'format_short_signature(signature)' = 'signature.username()'
 ```
 
+### Commit timestamp
+
+Commits have both an "author timestamp" and "committer timestamp". By default,
+jj displays the committer timestamp, but can be changed to show the author
+timestamp instead.
+
+The function must return a timestamp because the return value will likely be
+formatted with `format_timestamp()`.
+
+```toml
+[template-aliases]
+'commit_timestamp(commit)' = 'commit.author().timestamp()'
+```
+
 ### Allow "large" revsets by default
 
 Certain commands (such as `jj rebase`) can take multiple revset arguments, but