backend: allow negative timestamps in commits and operations

I was reading a draft of "Git Rev News: Edition 91" [1] where Peff
mentions some unfinished patches to allow negative timestamps in
Git. So I figured I should add support for that before I forget. I
haven't checked if libgit2 supports it, so it might be that our Git
backend still doesn't support it after this patch.

 [1] https://github.com/git/git.github.io/blob/master/rev_news/drafts/edition-91.md

lib/src/backend.rs

@@ -226,7 +226,7 @@ pub enum Phase {
 }
 
 #[derive(Debug, PartialEq, Eq, Clone, PartialOrd, Ord)]
-pub struct MillisSinceEpoch(pub u64);
+pub struct MillisSinceEpoch(pub i64);
 
 #[derive(Debug, PartialEq, Eq, Clone, PartialOrd, Ord)]
 pub struct Timestamp {
@@ -244,7 +244,7 @@ impl Timestamp {
         datetime: chrono::DateTime<Tz>,
     ) -> Self {
         Self {
-            timestamp: MillisSinceEpoch(datetime.timestamp_millis() as u64),
+            timestamp: MillisSinceEpoch(datetime.timestamp_millis()),
             tz_offset: datetime.offset().local_minus_utc() / 60,
         }
     }

lib/src/git_backend.rs

@@ -101,7 +101,7 @@ impl GitBackend {
 fn signature_from_git(signature: git2::Signature) -> Signature {
     let name = signature.name().unwrap_or("<no name>").to_owned();
     let email = signature.email().unwrap_or("<no email>").to_owned();
-    let timestamp = MillisSinceEpoch((signature.when().seconds() * 1000) as u64);
+    let timestamp = MillisSinceEpoch(signature.when().seconds() * 1000);
     let tz_offset = signature.when().offset_minutes();
     Signature {
         name,
@@ -117,7 +117,7 @@ fn signature_to_git(signature: &Signature) -> git2::Signature {
     let name = &signature.name;
     let email = &signature.email;
     let time = git2::Time::new(
-        (signature.timestamp.timestamp.0 / 1000) as i64,
+        signature.timestamp.timestamp.0.div_euclid(1000),
         signature.timestamp.tz_offset,
     );
     git2::Signature::new(name, email, &time).unwrap()

lib/src/protos/op_store.proto

@@ -78,7 +78,7 @@ message Operation {
 
 // TODO: Share with store.proto? Do we even need the timezone here?
 message Timestamp {
-  uint64 millis_since_epoch = 1;
+  int64 millis_since_epoch = 1;
   int32 tz_offset = 2;
 }
 

lib/src/protos/store.proto

@@ -47,7 +47,7 @@ message Commit {
   string description = 5;
 
   message Timestamp {
-    uint64 millis_since_epoch = 1;
+    int64 millis_since_epoch = 1;
     int32 tz_offset = 2;
   }
   message Signature {

lib/src/protos/working_copy.proto

@@ -24,7 +24,7 @@ enum FileType {
 }
 
 message FileState {
-  uint64 mtime_millis_since_epoch = 1;
+  int64 mtime_millis_since_epoch = 1;
   uint64 size = 2;
   FileType file_type = 3;
   // Set only if file_type is Conflict

lib/src/working_copy.rs

@@ -222,7 +222,8 @@ fn mtime_from_metadata(metadata: &Metadata) -> MillisSinceEpoch {
         .expect("mtime before unix epoch");
 
     MillisSinceEpoch(
-        u64::try_from(since_epoch.as_millis()).expect("mtime billions of years into the future"),
+        i64::try_from(since_epoch.as_millis())
+            .expect("mtime billions of years into the future or past"),
     )
 }
 

src/commands.rs

@@ -3685,8 +3685,8 @@ fn cmd_bench(
 fn format_timestamp(timestamp: &Timestamp) -> String {
     let utc = Utc
         .timestamp(
-            timestamp.timestamp.0 as i64 / 1000,
-            (timestamp.timestamp.0 % 1000) as u32 * 1000000,
+            timestamp.timestamp.0.div_euclid(1000),
+            (timestamp.timestamp.0.rem_euclid(1000)) as u32 * 1000000,
         )
         .with_timezone(&FixedOffset::east(timestamp.tz_offset * 60));
     utc.format("%Y-%m-%d %H:%M:%S.%3f %:z").to_string()

src/template_parser.rs

@@ -101,8 +101,8 @@ impl TemplateProperty<Signature, String> for SignatureTimestamp {
     fn extract(&self, context: &Signature) -> String {
         let utc = Utc
             .timestamp(
-                context.timestamp.timestamp.0 as i64 / 1000,
-                (context.timestamp.timestamp.0 % 1000) as u32 * 1000000,
+                context.timestamp.timestamp.0.div_euclid(1000),
+                context.timestamp.timestamp.0.rem_euclid(1000) as u32 * 1000000,
             )
             .with_timezone(&FixedOffset::east(context.timestamp.tz_offset * 60));
         utc.format("%Y-%m-%d %H:%M:%S.%3f %:z").to_string()