diff --git a/crates/engine/src/io.rs b/crates/engine/src/io.rs
index ba44f1f2b..3b8bacfe4 100644
--- a/crates/engine/src/io.rs
+++ b/crates/engine/src/io.rs
@@ -301,12 +301,9 @@ pub fn csv_values_bytes(wb: &Workbook) -> Result<Vec<u8>> {
     let Some(sh) = wb.sheet(wb.active_sheet()) else { return Ok(Vec::new()) };
     gridcraft_xlsx::write_csv(sh, wb.csv_delimiter.unwrap_or(b','), |c| match sh.value(c) {
         gridcraft_core::Value::Number(n) if n.is_finite() => {
-            let kind = crate::display::number_format(wb.styles.get(sh.style_id(c)).num_fmt.as_str()).kind();
-            let dt = matches!(kind, gridcraft_numfmt::FormatKind::Date | gridcraft_numfmt::FormatKind::Time)
-                .then(|| gridcraft_core::date::datetime_from_serial(wb.date_system, n))
-                .flatten();
-            match dt {
-                Some(d) => iso(&d, kind == gridcraft_numfmt::FormatKind::Time && n < 1.0, n.fract() != 0.0),
+            let shape = crate::display::number_format(wb.styles.get(sh.style_id(c)).num_fmt.as_str()).date_shape();
+            match shape.zip(gridcraft_core::date::datetime_from_serial(wb.date_system, n)) {
+                Some(((date, time), d)) => iso(&d, date, time),
                 None => n.to_string(),
             }
         }
@@ -316,16 +313,17 @@ pub fn csv_values_bytes(wb: &Workbook) -> Result<Vec<u8>> {
     .map_err(|e| EngineError::Other(e.to_string()))
 }
 
-fn iso(d: &gridcraft_core::date::DateTime, time_only: bool, with_time: bool) -> String {
-    let date = format!("{:04}-{:02}-{:02}", d.year, d.month, d.day);
-    let mut time = format!("{:02}:{:02}:{:02}", d.hour, d.minute, d.second);
+/// `date` and `time` say which parts the cell's format shows; a format with neither is not a date.
+fn iso(d: &gridcraft_core::date::DateTime, date: bool, time: bool) -> String {
+    let day = format!("{:04}-{:02}-{:02}", d.year, d.month, d.day);
+    let mut clock = format!("{:02}:{:02}:{:02}", d.hour, d.minute, d.second);
     if d.milli != 0 {
-        time.push_str(&format!(".{:03}", d.milli));
+        clock.push_str(&format!(".{:03}", d.milli));
     }
-    match (time_only, with_time) {
-        (true, _) => time,
-        (false, true) => format!("{date}T{time}"),
-        (false, false) => date,
+    match (date, time) {
+        (true, true) => format!("{day}T{clock}"),
+        (false, true) => clock,
+        _ => day,
     }
 }
 
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index c42d1484b..c76320592 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -2463,6 +2463,7 @@ fn saving_an_opened_semicolon_csv_keeps_semicolons() {
 /// `file.exportCsv` writes displayed text by default (Excel's Save As CSV); `values: true` writes the
 /// raw values for automation: full precision, ISO dates and times.
 #[test]
+#[cfg(not(target_arch = "wasm32"))]
 fn export_csv_values_writes_raw_values_and_the_default_stays_displayed_text() {
     let mut s = s();
     for (cell, input, code) in [
@@ -2474,6 +2475,10 @@ fn export_csv_values_writes_raw_values_and_the_default_stays_displayed_text() {
         ("F1", "TRUE", "General"),
         ("G1", "a,b", "General"),
         ("H1", "=1/0", "General"),
+        ("I1", "1", "h:mm"),
+        ("J1", "1.5", "h:mm"),
+        ("K1", "46023", "yyyy-mm-dd hh:mm"),
+        ("L1", "1.5", "[h]:mm"),
     ] {
         s.execute("cell.set", json!({"cell": cell, "input": input})).unwrap();
         s.execute("home.numberFormat", json!({"range": cell, "code": code})).unwrap();
@@ -2494,7 +2499,10 @@ fn export_csv_values_writes_raw_values_and_the_default_stays_displayed_text() {
     assert_eq!(std::fs::read(dir.join("off.csv")).unwrap(), default);
     let default = String::from_utf8(default).unwrap();
     assert!(default.starts_with("\"$1,234.50\",0.12,1-Jan-26,"), "{default}");
-    assert_eq!(std::fs::read_to_string(&raw).unwrap(), "1234.5,0.123456789,2026-01-01,2026-01-01T12:00:00,18:00:00,TRUE,\"a,b\",#DIV/0!\r\n");
+    assert_eq!(
+        std::fs::read_to_string(&raw).unwrap(),
+        "1234.5,0.123456789,2026-01-01,2026-01-01T12:00:00,18:00:00,TRUE,\"a,b\",#DIV/0!,00:00:00,12:00:00,2026-01-01T00:00:00,1.5\r\n"
+    );
     std::fs::remove_dir_all(dir).unwrap();
 }
 
diff --git a/crates/numfmt/src/lib.rs b/crates/numfmt/src/lib.rs
index 06d0a8693..d5c4ef41b 100644
--- a/crates/numfmt/src/lib.rs
+++ b/crates/numfmt/src/lib.rs
@@ -15,7 +15,7 @@ use gridcraft_core::{CellError, DateSystem, Value};
 pub use builtin::{builtin_format, builtin_id};
 pub use decimal::format_general_fit;
 
-use parse::{SecKind, Section, Tok};
+use parse::{DatePart, SecKind, Section, Tok};
 use render::Out;
 
 /// Width (in characters) of General output in a cell.
@@ -193,6 +193,18 @@ impl NumberFormat {
         self.num_sections().iter().any(|s| s.kind == SecKind::Date)
     }
 
+    /// For a date or time format, whether its first section shows calendar parts (year, month,
+    /// day) and whether it shows time parts, as `(date, time)`. `None` for any other format and for
+    /// elapsed-time codes like `[h]:mm`, whose value is a duration, not a moment.
+    pub fn date_shape(&self) -> Option<(bool, bool)> {
+        let s = self.num_sections().first().filter(|s| s.kind == SecKind::Date)?;
+        let parts = || s.toks.iter().filter_map(|t| if let Tok::Date(d) = t { Some(*d) } else { None });
+        if parts().any(|d| matches!(d, DatePart::ElapsedH(_) | DatePart::ElapsedM(_) | DatePart::ElapsedS(_))) {
+            return None;
+        }
+        Some((parts().any(DatePart::is_calendar), parts().any(|d| !d.is_calendar())))
+    }
+
     pub fn is_percent(&self) -> bool {
         self.num_sections().iter().any(|s| s.kind == SecKind::Number && s.percent > 0)
     }