diff --git a/static/exporter.js b/static/exporter.js
index 23535d8..06dd2e6 100644
--- a/static/exporter.js
+++ b/static/exporter.js
@@ -112,11 +112,24 @@ function _instrCharacterLines(instr) {
     });
 }
 
+// RFC §4.4.1: DATE = YYYY-MM-DD HH:MM:SS
+// notChangedSince from AST log is a float epoch; score YAML must have ISO date.
+
+function _epochToISO(val) {
+    if (!val) return null;
+    if (typeof val === 'string') return val;
+    return new Date(val * 1000).toISOString().slice(0, 19).replace('T', ' ');
+}
+
+function _nowISO() {
+    return new Date().toISOString().slice(0, 19).replace('T', ' ');
+}
+
 // RFC §4.4: embedded instrument key is "instrument NAME:" not "instrument: 'NAME'"
 
 export function exportInstrument(instr) {
     const lines = [`instrument ${instr.name}:`];
-    if (instr.notChangedSince) lines.push(`  NOT_CHANGED_SINCE: ${instr.notChangedSince}`);
+    lines.push(`  NOT_CHANGED_SINCE: ${_nowISO()}`);
     lines.push(`  character:`);
     lines.push(..._instrCharacterLines(instr));
     return lines.join('\n');
@@ -210,6 +223,13 @@ function _patchInstrumentHeader(text, instruments) {
             } else if (instr.isLinked && !instr._modified) {
                 result.push(line);
                 i++;
+                while (i < lines.length && (lines[i].startsWith(' ') || lines[i] === '')) {
+                    if (/^\s+NOT_CHANGED_SINCE\s*:/.test(lines[i]))
+                        result.push(`  NOT_CHANGED_SINCE: ${_epochToISO(instr.notChangedSince)}`);
+                    else
+                        result.push(lines[i]);
+                    i++;
+                }
             } else {
                 i++;
                 while (i < lines.length && (lines[i].startsWith(' ') || lines[i] === '')) i++;
diff --git a/test-parser.mjs b/test-parser.mjs
index 406beff..5b5b119 100644
--- a/test-parser.mjs
+++ b/test-parser.mjs
@@ -542,6 +542,23 @@ ok('new bar has _meta with BPM', newBarPatched.includes('beats_per_minute: 120')
 ok('new bar appended as extra document', (newBarPatched.match(/\n---\n/g) ?? []).length === 3);
 ok('new bar logged as changed', newBarLog.some(e => e.level === 'changed' && e.path === 'bar / 001P1L1M3'));
 
+// ── NOT_CHANGED_SINCE epoch → ISO conversion ──────────────────────────────
+section('NOT_CHANGED_SINCE: epoch converted on passthrough, now on re-serialize');
+const ncsScore = `instrument linked/pi:\n  NOT_CHANGED_SINCE: 1710000000.0\n  character:\n    A: "1:0,10;1,0"\n\ninstrument embedded:\n  NOT_CHANGED_SINCE: 1710000000.0\n  character:\n    A: "1:0,5;1,0"\n`;
+const ncsList = [
+    { name: 'linked/pi', isLinked: true, _modified: false, notChangedSince: 1710000000.0,
+      variations: [], basicProperties: null, volumes: null, timbre: null, fmModulations: [], amModulations: [] },
+    { name: 'embedded', isLinked: false, _modified: true, notChangedSince: 1710000000.0,
+      variations: [], basicProperties: null, volumes: null, timbre: null, fmModulations: [], amModulations: [] },
+];
+const { text: ncsPatched } = patchScore(ncsScore, ncsList);
+const ISO_RX = /\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2}/;
+const ncsLines = ncsPatched.split('\n');
+const linkedNCS = ncsLines.find(l => /NOT_CHANGED_SINCE/.test(l) && ncsLines.indexOf(l) < ncsLines.findIndex(l2 => /instrument embedded/.test(l2)));
+const embeddedNCS = ncsLines.find((l, i) => /NOT_CHANGED_SINCE/.test(l) && i > ncsLines.findIndex(l2 => /instrument embedded/.test(l2)));
+ok('linked passthrough: epoch converted to ISO date', linkedNCS && ISO_RX.test(linkedNCS) && !linkedNCS.includes('1710000000'));
+ok('modified instrument: NOT_CHANGED_SINCE set to now', embeddedNCS && ISO_RX.test(embeddedNCS) && !embeddedNCS.includes('1710000000'));
+
 // ── patchScore deleted instrument ─────────────────────────────────────────
 section('patchScore deleted instrument');
 const delInstrScore = `instrument alpha:\n  character:\n    A: "1:0,10;1,0"\n\ninstrument ki:\n  character:\n    A: "1:0,5;1,0"\n`;