From 27be1753c2d8c1284498a46b4547ccc4d7d7a83c Mon Sep 17 00:00:00 2001
From: MarekSpodBiedry <73122968+MarekSpodBiedry@users.noreply.github.com>
Date: Sat, 10 Oct 2026 23:43:32 +0200
Subject: [PATCH] Read the browser clock for NOW(), TODAY() and year-less dates
 on wasm (#202)

---
 Cargo.lock                |  1 +
 crates/calc/src/recalc.rs | 14 ++++----------
 crates/core/Cargo.toml    |  3 +++
 crates/core/src/date.rs   | 28 ++++++++++++++++++++++++++++
 crates/core/src/parse.rs  | 14 +++++---------
 5 files changed, 41 insertions(+), 19 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index 5dce47768..e1d8d3e5e 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1469,6 +1469,7 @@ dependencies = [
 name = "gridcraft-core"
 version = "0.4.0"
 dependencies = [
+ "js-sys",
  "serde",
  "serde_json",
  "thiserror 2.0.21",
diff --git a/crates/calc/src/recalc.rs b/crates/calc/src/recalc.rs
index 4cf0c6a21..9df724573 100644
--- a/crates/calc/src/recalc.rs
+++ b/crates/calc/src/recalc.rs
@@ -919,17 +919,11 @@ impl Calc {
     }
 }
 
-/// Local date-time as a serial (1900 system). Wasm without a clock returns a fixed date.
-#[allow(clippy::disallowed_methods)] // the clock is read only off wasm
+/// Current date-time as a serial (1900 system), from the system clock (browser clock on wasm).
+/// UTC; the local offset isn't available without platform calls. Falls back to a fixed date only
+/// when no clock is available.
 pub fn now_serial() -> f64 {
-    #[cfg(not(target_arch = "wasm32"))]
-    {
-        if let Ok(d) = std::time::SystemTime::now().duration_since(std::time::UNIX_EPOCH) {
-            // UTC; local offset isn't available without platform calls.
-            return 25569.0 + d.as_secs_f64() / 86400.0;
-        }
-    }
-    46302.5
+    gridcraft_core::date::unix_now_ms().map_or(46302.5, gridcraft_core::date::serial_from_unix_ms)
 }
 
 /// Evaluates a formula in the context of a cell without storing it (conditional formats, data
diff --git a/crates/core/Cargo.toml b/crates/core/Cargo.toml
index 26cd19f08..1decf6257 100644
--- a/crates/core/Cargo.toml
+++ b/crates/core/Cargo.toml
@@ -12,5 +12,8 @@ workspace = true
 serde = { workspace = true }
 thiserror = { workspace = true }
 
+[target.'cfg(target_arch = "wasm32")'.dependencies]
+js-sys = "0.3"
+
 [dev-dependencies]
 serde_json = { workspace = true }
diff --git a/crates/core/src/date.rs b/crates/core/src/date.rs
index 81fa6c55c..3b126b7fd 100644
--- a/crates/core/src/date.rs
+++ b/crates/core/src/date.rs
@@ -39,6 +39,26 @@ pub fn days_from_civil(y: i32, m: u32, d: u32) -> i64 {
     era * 146097 + doe - 719468
 }
 
+/// Milliseconds since the Unix epoch (UTC) from the system clock; `None` when the clock is unavailable.
+/// On wasm32 `std::time::SystemTime::now` traps, so the browser clock is read through `Date.now()`.
+#[allow(clippy::disallowed_methods)] // the std clock is read only off wasm
+pub fn unix_now_ms() -> Option<f64> {
+    #[cfg(not(target_arch = "wasm32"))]
+    {
+        let d = std::time::SystemTime::now().duration_since(std::time::UNIX_EPOCH).ok()?;
+        Some(d.as_secs_f64() * 1000.0)
+    }
+    #[cfg(target_arch = "wasm32")]
+    {
+        Some(js_sys::Date::now())
+    }
+}
+
+/// Date serial (1900 system, with fraction) for a Unix time in milliseconds.
+pub fn serial_from_unix_ms(ms: f64) -> f64 {
+    25569.0 + ms / 86_400_000.0
+}
+
 pub fn civil_from_days(z: i64) -> (i32, u32, u32) {
     let z = z + 719468;
     let era = if z >= 0 { z } else { z - 146096 } / 146097;
@@ -160,6 +180,14 @@ pub const WEEKDAYS: [&str; 7] = ["Sunday", "Monday", "Tuesday", "Wednesday", "Th
 mod tests {
     use super::*;
 
+    #[test]
+    fn serial_from_unix_time() {
+        assert_eq!(serial_from_unix_ms(0.0), 25569.0);
+        // 2026-10-10T12:00:00Z
+        assert_eq!(serial_from_unix_ms(1_791_633_600_000.0), 46305.5);
+        assert!(unix_now_ms().is_some_and(|ms| serial_from_unix_ms(ms) > 46305.0));
+    }
+
     #[test]
     fn known_serials() {
         let s = DateSystem::D1900;
diff --git a/crates/core/src/parse.rs b/crates/core/src/parse.rs
index 7169ae545..b0ed80e07 100644
--- a/crates/core/src/parse.rs
+++ b/crates/core/src/parse.rs
@@ -273,16 +273,12 @@ fn valid_ymd(sys: DateSystem, y: i64, m: i64, d: i64) -> Option<f64> {
     serial_from_ymd(sys, y, m, d)
 }
 
-/// The year used for dates typed without one. Fixed per process from the system clock when
-/// available (wasm without clock falls back to 2026).
-#[allow(clippy::disallowed_methods)] // the clock is read only off wasm
+/// The year used for dates typed without one, from the system clock (browser clock on wasm).
+/// Falls back to 2026 only when no clock is available.
 pub fn current_year() -> i64 {
-    #[cfg(not(target_arch = "wasm32"))]
-    {
-        if let Ok(d) = std::time::SystemTime::now().duration_since(std::time::UNIX_EPOCH) {
-            let days = (d.as_secs() / 86400) as i64;
-            return crate::date::civil_from_days(days).0 as i64;
-        }
+    if let Some(ms) = crate::date::unix_now_ms() {
+        let days = (ms / 86_400_000.0).floor() as i64;
+        return crate::date::civil_from_days(days).0 as i64;
     }
     2026
 }