diff --git a/src/layers/shipwrecks/period.js b/src/layers/shipwrecks/period.js
index f0cfa26e2..33b7885db 100644
--- a/src/layers/shipwrecks/period.js
+++ b/src/layers/shipwrecks/period.js
@@ -29,7 +29,7 @@ function yearParam(value) {
 /**
  * Merge a (partial) period request into the current one. `from: null` means
  * the first year in the data and `to: null` the latest; an invalid year keeps
- * the current one.
+ * the current one, and a start after the end is stored as the reverse.
  * @param {{from?: number|string|null, to?: number|string|null}} params
  * @param {{from: number|null, to: number|null}} current
  * @returns {{from: number|null, to: number|null}}
@@ -44,6 +44,10 @@ export function normalizePeriod(
     const year = yearParam(params[end]);
     if (year !== undefined) next[end] = year;
   }
+  // Stored in the order it is shown, so a later edit to one end (the picker
+  // sends only that end) changes the end the reader picked, not the other.
+  if (Number.isInteger(next.from) && Number.isInteger(next.to))
+    if (next.from > next.to) [next.from, next.to] = [next.to, next.from];
   return next;
 }
 
diff --git a/src/layers/shipwrecks/period.test.mjs b/src/layers/shipwrecks/period.test.mjs
index 7839a2840..80c9eb866 100644
--- a/src/layers/shipwrecks/period.test.mjs
+++ b/src/layers/shipwrecks/period.test.mjs
@@ -36,3 +36,23 @@ test('all years show every wreck; a narrower period hides undated ones', () => {
   assert.deepEqual(reversed, { from: 1900, to: 1950, all: false });
   assert.equal(inPeriod(RECORDS[1], reversed), true);
 });
+
+test('a reversed period is stored as shown, so editing one end keeps the other', () => {
+  const reversed = normalizePeriod({ from: 1950, to: 1900 });
+  assert.deepEqual(reversed, { from: 1900, to: 1950 });
+  // The picker shows 1900–1950; the reader changes "From" to 1920.
+  assert.deepEqual(normalizePeriod({ from: 1920 }, reversed), {
+    from: 1920,
+    to: 1950,
+  });
+  // Moving "From" past "To" reads as the reverse, never an empty range.
+  assert.deepEqual(normalizePeriod({ from: 1960 }, { from: 1900, to: 1950 }), {
+    from: 1950,
+    to: 1960,
+  });
+  // One open end stays open.
+  assert.deepEqual(normalizePeriod({ from: 1960 }, { from: null, to: null }), {
+    from: 1960,
+    to: null,
+  });
+});