Read SOURCE_DATE_EPOCH for better reproducibility (#3809)

crates/typst-cli/Cargo.toml

@@ -62,6 +62,7 @@ zip = { workspace = true, optional = true }
 openssl = { workspace = true }
 
 [build-dependencies]
+chrono = { workspace = true }
 clap = { workspace = true, features = ["string"] }
 clap_complete = { workspace = true }
 clap_mangen = { workspace = true }

crates/typst-cli/src/args.rs

@@ -1,6 +1,7 @@
 use std::fmt::{self, Display, Formatter};
 use std::path::PathBuf;
 
+use chrono::{DateTime, Utc};
 use clap::builder::ValueParser;
 use clap::{ArgAction, Args, ColorChoice, Parser, Subcommand, ValueEnum};
 use semver::Version;
@@ -167,6 +168,12 @@ pub struct SharedArgs {
     )]
     pub font_paths: Vec<PathBuf>,
 
+    /// The document's creation date formatted as a UNIX timestamp.
+    ///
+    /// For more information, see <https://reproducible-builds.org/specs/source-date-epoch/>.
+    #[clap(env = "SOURCE_DATE_EPOCH", value_parser = parse_source_date_epoch)]
+    pub source_date_epoch: Option<DateTime<Utc>>,
+
     /// The format to emit diagnostics in
     #[clap(
         long,
@@ -176,6 +183,15 @@ pub struct SharedArgs {
     pub diagnostic_format: DiagnosticFormat,
 }
 
+/// Parses a UNIX timestamp according to <https://reproducible-builds.org/specs/source-date-epoch/>
+fn parse_source_date_epoch(raw: &str) -> Result<DateTime<Utc>, String> {
+    let timestamp: i64 = raw
+        .parse()
+        .map_err(|err| format!("timestamp must be decimal integer ({err})"))?;
+    DateTime::from_timestamp(timestamp, 0)
+        .ok_or_else(|| "timestamp out of range".to_string())
+}
+
 /// An input that is either stdin or a real path.
 #[derive(Debug, Clone)]
 pub enum Input {

crates/typst-cli/src/compile.rs

@@ -166,7 +166,10 @@ fn export(
 
 /// Export to a PDF.
 fn export_pdf(document: &Document, command: &CompileCommand) -> StrResult<()> {
-    let buffer = typst_pdf::pdf(document, Smart::Auto, now());
+    let timestamp = convert_datetime(
+        command.common.source_date_epoch.unwrap_or_else(chrono::Utc::now),
+    );
+    let buffer = typst_pdf::pdf(document, Smart::Auto, timestamp);
     command
         .output()
         .write(&buffer)
@@ -174,16 +177,15 @@ fn export_pdf(document: &Document, command: &CompileCommand) -> StrResult<()> {
     Ok(())
 }
 
-/// Get the current date and time in UTC.
-fn now() -> Option<Datetime> {
-    let now = chrono::Local::now().naive_utc();
+/// Convert [`chrono::DateTime`] to [`Datetime`]
+fn convert_datetime(date_time: chrono::DateTime<chrono::Utc>) -> Option<Datetime> {
     Datetime::from_ymd_hms(
-        now.year(),
-        now.month().try_into().ok()?,
-        now.day().try_into().ok()?,
-        now.hour().try_into().ok()?,
-        now.minute().try_into().ok()?,
-        now.second().try_into().ok()?,
+        date_time.year(),
+        date_time.month().try_into().ok()?,
+        date_time.day().try_into().ok()?,
+        date_time.hour().try_into().ok()?,
+        date_time.minute().try_into().ok()?,
+        date_time.second().try_into().ok()?,
     )
 }
 

crates/typst-cli/src/world.rs

@@ -4,7 +4,7 @@ use std::path::{Path, PathBuf};
 use std::sync::OnceLock;
 use std::{fmt, fs, io, mem};
 
-use chrono::{DateTime, Datelike, Local};
+use chrono::{DateTime, Datelike, FixedOffset, Local, Utc};
 use comemo::Prehashed;
 use ecow::{eco_format, EcoString};
 use once_cell::sync::Lazy;
@@ -42,8 +42,9 @@ pub struct SystemWorld {
     /// Maps file ids to source files and buffers.
     slots: Mutex<HashMap<FileId, FileSlot>>,
     /// The current datetime if requested. This is stored here to ensure it is
-    /// always the same within one compilation. Reset between compilations.
-    now: OnceLock<DateTime<Local>>,
+    /// always the same within one compilation.
+    /// Reset between compilations if not [`Now::Fixed`].
+    now: Now,
     /// The export cache, used for caching output files in `typst watch`
     /// sessions.
     export_cache: ExportCache,
@@ -104,6 +105,11 @@ impl SystemWorld {
         let mut searcher = FontSearcher::new();
         searcher.search(&command.font_paths);
 
+        let now = match command.source_date_epoch {
+            Some(time) => Now::Fixed(time),
+            None => Now::System(OnceLock::new()),
+        };
+
         Ok(Self {
             workdir: std::env::current_dir().ok(),
             root,
@@ -112,7 +118,7 @@ impl SystemWorld {
             book: Prehashed::new(searcher.book),
             fonts: searcher.fonts,
             slots: Mutex::new(HashMap::new()),
-            now: OnceLock::new(),
+            now,
             export_cache: ExportCache::new(),
         })
     }
@@ -146,7 +152,9 @@ impl SystemWorld {
         for slot in self.slots.get_mut().values_mut() {
             slot.reset();
         }
-        self.now.take();
+        if let Now::System(time_lock) = &mut self.now {
+            time_lock.take();
+        }
     }
 
     /// Lookup a source file by id.
@@ -187,17 +195,24 @@ impl World for SystemWorld {
     }
 
     fn today(&self, offset: Option<i64>) -> Option<Datetime> {
-        let now = self.now.get_or_init(chrono::Local::now);
+        let now = match &self.now {
+            Now::Fixed(time) => time,
+            Now::System(time) => time.get_or_init(Utc::now),
+        };
 
-        let naive = match offset {
-            None => now.naive_local(),
-            Some(o) => now.naive_utc() + chrono::Duration::try_hours(o)?,
+        // The time with the specified UTC offset, or within the local time zone.
+        let with_offset = match offset {
+            None => now.with_timezone(&Local).fixed_offset(),
+            Some(hours) => {
+                let seconds = i32::try_from(hours).ok()?.checked_mul(3600)?;
+                now.with_timezone(&FixedOffset::east_opt(seconds)?)
+            }
         };
 
         Datetime::from_ymd(
-            naive.year(),
-            naive.month().try_into().ok()?,
-            naive.day().try_into().ok()?,
+            with_offset.year(),
+            with_offset.month().try_into().ok()?,
+            with_offset.day().try_into().ok()?,
         )
     }
 }
@@ -384,6 +399,15 @@ fn decode_utf8(buf: &[u8]) -> FileResult<&str> {
     Ok(std::str::from_utf8(buf.strip_prefix(b"\xef\xbb\xbf").unwrap_or(buf))?)
 }
 
+/// The current date and time.
+enum Now {
+    /// The date and time if the environment `SOURCE_DATE_EPOCH` is set.
+    /// Used for reproducible builds.
+    Fixed(DateTime<Utc>),
+    /// The current date and time if the time is not externally fixed.
+    System(OnceLock<DateTime<Utc>>),
+}
+
 /// An error that occurs during world construction.
 #[derive(Debug)]
 pub enum WorldCreationError {