fix: remove system time deps from crates (#1621)

* fix: remove system time deps from crates

* fix: remove system time deps from crates

* fix: smater feature gate

* docs: add some todos

* Update time.rs

* Update Cargo.toml

* build: remove hard dep chrono

crates/tinymist-std/src/time.rs

@@ -1,8 +1,18 @@
 //! Cross platform time utilities.
 
 pub use std::time::SystemTime as Time;
+pub use time::UtcDateTime;
+
+#[cfg(not(feature = "web"))]
+pub use std::time::{Duration, Instant};
+#[cfg(feature = "web")]
 pub use web_time::{Duration, Instant};
 
+/// Returns the current datetime in utc (UTC+0).
+pub fn utc_now() -> UtcDateTime {
+    now().into()
+}
+
 /// Returns the current system time (UTC+0).
 #[cfg(any(feature = "system", feature = "web"))]
 pub fn now() -> Time {
@@ -22,3 +32,30 @@ pub fn now() -> Time {
 pub fn now() -> Time {
     Time::UNIX_EPOCH
 }
+
+/// The trait helping convert to a [`UtcDateTime`].
+pub trait ToUtcDateTime {
+    /// Converts to a [`UtcDateTime`].
+    fn to_utc_datetime(self) -> Option<UtcDateTime>;
+}
+
+impl ToUtcDateTime for i64 {
+    /// Converts a UNIX timestamp to a [`UtcDateTime`].
+    fn to_utc_datetime(self) -> Option<UtcDateTime> {
+        UtcDateTime::from_unix_timestamp(self).ok()
+    }
+}
+
+impl ToUtcDateTime for Time {
+    /// Converts a system time to a [`UtcDateTime`].
+    fn to_utc_datetime(self) -> Option<UtcDateTime> {
+        Some(UtcDateTime::from(self))
+    }
+}
+
+/// Converts a [`UtcDateTime`] to typst's datetime.
+#[cfg(feature = "typst")]
+pub fn to_typst_time(timestamp: UtcDateTime) -> typst::foundations::Datetime {
+    let datetime = ::time::PrimitiveDateTime::new(timestamp.date(), timestamp.time());
+    typst::foundations::Datetime::Datetime(datetime)
+}