diff --git a/CLAUDE.md b/CLAUDE.md
index c2cf090..3c60247 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -961,10 +961,16 @@ column.
   which is possible but is inference, not recovery — deleting and replaying
   those log entries is the honest repair
 
-- **Zero-row operations report success.** Scale refuses with "Nothing to
-  scale…" when its slice matches nothing; recode and clone commit an empty log
-  entry and return `rows_affected: 0`. A recode of a rep whose rows are all
-  `reference` looked like it worked and did nothing
+- **Zero-row clones report success.** Scale refuses with "Nothing to scale…"
+  and recode with "Nothing to recode…" when there is nothing to write (recode
+  also skips coordinates that already net to zero, rather than writing an empty
+  pair for each); clone still commits an empty log entry and returns
+  `rows_affected: 0`
+- **A date offset must be whole days.** `assertInterval()` refuses anything
+  finer, because every shifted date is cast back to a date — and Postgres reads
+  a bare number as *seconds*, so `"30"` used to be accepted and move nothing.
+  `30 days` is not a month either: a row on 1 October lands on the 31st; use
+  `1 month` to move a month into the next
 - **The change log does not show an entry's id**, so there is no way to name
   one when asking about it
 - **Territory is read onto the session at login**, so granting or changing one
diff --git a/lib/sql_generator.js b/lib/sql_generator.js
index 40de10c..b993705 100644
--- a/lib/sql_generator.js
+++ b/lib/sql_generator.js
@@ -439,6 +439,11 @@ ilog AS (
     {{exclude_clause}}
     GROUP BY
         ${groupCols([...dims, ...dateCols])}
+    -- a coordinate whose earlier adjustments already net it to zero has nothing to
+    -- move; writing it anyway is two empty rows per coordinate. Rare on a first
+    -- recode, but a second recode over cells a first one emptied would rewrite
+    -- every one of them
+    HAVING ${[effectiveValue, effectiveUnits].filter(Boolean).map(c => `sum(${q(c)}) <> 0`).join(' OR ')}
 )
 ,neg AS (
     INSERT INTO {{fc_table}} (${insertCols})
diff --git a/routes/operations.js b/routes/operations.js
index 74223b9..1c11dea 100644
--- a/routes/operations.js
+++ b/routes/operations.js
@@ -97,17 +97,30 @@ module.exports = function(pool) {
     // Postgres to parse it alone first, where the failure is cheap and can name the
     // field it came from. Negative intervals are valid and useful -- '-90 days'
     // pulls a plan back a quarter -- so this checks validity, not sign.
+    // A date offset has to be a whole number of days. Every date it shifts is
+    // cast back to a date, so anything finer is rounded away -- and Postgres
+    // reads a bare number as seconds, so "30" meant thirty seconds, moved
+    // nothing, and a shifted recode took rows out of a cell and put them back
+    // into the same one.
     async function assertInterval(value, res) {
+        const example = `Try something like "30 days", "1 month", "-90 days" or "0 days".`;
+        let whole;
         try {
-            await pool.query(`SELECT $1::interval`, [value]);
-            return true;
+            whole = (await pool.query(
+                `SELECT date_trunc('day', $1::interval) = $1::interval AS whole`, [value]
+            )).rows[0].whole;
         } catch {
+            res.status(400).json({ error: `"${value}" is not a valid interval. ${example}` });
+            return false;
+        }
+        if (!whole) {
             res.status(400).json({
-                error: `"${value}" is not a valid interval. Try something like `
-                     + `"4 months", "1 year", "-90 days" or "0 days".`
+                error: `"${value}" is not a whole number of days, so it would not move any date `
+                     + `(a number with no unit is read as seconds). ${example}`
             });
             return false;
         }
+        return true;
     }
 
     // A slice is only meaningful if at least one of its keys is a filterable
@@ -977,6 +990,16 @@ module.exports = function(pool) {
                     }
                     allRows.push(...result.rows);
                 }
+                // Every coordinate in the selection nets to zero -- nothing to move.
+                // Refused like scale's no-op rather than committing an empty log
+                // entry that reads as a recode that happened.
+                if (allRows.length === 0) {
+                    await client.query('ROLLBACK');
+                    return res.status(400).json({
+                        error: 'Nothing to recode — the selection nets to zero, or holds only rows recode cannot touch'
+                    });
+                }
+
                 await client.query('COMMIT');
                 committed = true;
                 // one log id per unit: apply_mode 'each' writes an entry per slice