From 8d6254aa0b324677ecd2ec2c063b8a2febf39d22 Mon Sep 17 00:00:00 2001
From: NetterSilver <133535268+Silvertree2010@users.noreply.github.com>
Date: Sun, 18 Oct 2026 23:06:18 +0200
Subject: [PATCH] Render weekday letters in FormCalc date pictures (#3321)

Found while testing crates/js/src/formcalc.rs:1929-1957 (format_date), with DateFmt returning "EEEE, MMMM D, YYYY" at :2333

Co-authored-by: Claude Haiku 5.5 <noreply@anthropic.com>
---
 crates/js/src/formcalc.rs | 15 +++++++++++++++
 crates/js/src/tests.rs    | 15 +++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/crates/js/src/formcalc.rs b/crates/js/src/formcalc.rs
index cf82eff46..dc32d457b 100644
--- a/crates/js/src/formcalc.rs
+++ b/crates/js/src/formcalc.rs
@@ -1933,6 +1933,8 @@ fn days_in(y: i64, m: u32) -> u32 {
     }
 }
 
+const WEEKDAYS: [&str; 7] = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"];
+
 fn format_date(y: i64, m: u32, d: u32, picture: &str) -> String {
     let pic: Vec<char> = picture.chars().collect();
     let mut out = String::new();
@@ -1955,6 +1957,19 @@ fn format_date(y: i64, m: u32, d: u32, picture: &str) -> String {
                 ('D', 1) => out.push_str(&d.to_string()),
                 (_, _) => out.push_str(&format!("{d:02}")),
             }
+        } else if pic[i] == 'E' {
+            let mut run = 1;
+            while i + run < pic.len() && pic[i + run] == 'E' {
+                run += 1;
+            }
+            i += run;
+            // Day 1 (1900-01-01) was a Monday, so the serial modulo 7 indexes from Sunday.
+            let weekday = days_from_civil(y, m, d).rem_euclid(7) as usize;
+            match run {
+                1 | 2 => out.push_str(&(weekday + 1).to_string()),
+                3 => out.push_str(WEEKDAYS.get(weekday).map_or("", |n| &n[..3])),
+                _ => out.push_str(WEEKDAYS.get(weekday).copied().unwrap_or("")),
+            }
         } else {
             out.push(pic[i]);
             i += 1;
diff --git a/crates/js/src/tests.rs b/crates/js/src/tests.rs
index 4fe5fd290..61d3a71e5 100644
--- a/crates/js/src/tests.rs
+++ b/crates/js/src/tests.rs
@@ -636,6 +636,21 @@ mod formcalc {
         assert_eq!(calc("; a comment\n// another\nSum(1,\n 2,\n 3)"), "6");
     }
 
+    #[test]
+    fn date_pictures_render_the_weekday_letters() {
+        assert_eq!(calc(r#"Num2Date(45350, DateFmt(4))"#), "Thursday, February 29, 2024");
+        assert_eq!(calc(r#"Num2Date(45350, "EEE, YYYY-MM-DD")"#), "Thu, 2024-02-29");
+        assert_eq!(calc(r#"Num2Date(45350, "EEEE")"#), "Thursday");
+        assert_eq!(calc(r#"Num2Date(45350, "E")"#), "5");
+        assert_eq!(calc(r#"Num2Date(45351, "EEEE")"#), "Friday");
+        assert_eq!(calc(r#"Num2Date(45352, "EEEE")"#), "Saturday");
+        assert_eq!(calc(r#"Num2Date(45353, "EEEE")"#), "Sunday");
+        assert_eq!(calc(r#"Num2Date(1, "EEEE")"#), "Monday");
+        assert_eq!(calc(r#"Format("EEEE, YYYY-MM-DD", 45350)"#), "Thursday, 2024-02-29");
+        // Other letters are still literal text.
+        assert_eq!(calc(r#"Num2Date(45350, "YYYY-MM-DD x")"#), "2024-02-29 x");
+    }
+
     #[test]
     fn str_and_format_round_exact_ties_away_from_zero() {
         assert_eq!(calc("Str(2.5, 3, 0)"), "  3");