Reset extraNonce only when prevBlock changes, so miners can continue updating the time on their work until it's stale

main.cpp

@@ -3447,12 +3447,13 @@ CBlock* CreateNewBlock(CReserveKey& reservekey)
 void IncrementExtraNonce(CBlock* pblock, CBlockIndex* pindexPrev, unsigned int& nExtraNonce, int64& nPrevTime)
 {
     // Update nExtraNonce
-    int64 nNow = max(pindexPrev->GetMedianTimePast()+1, GetAdjustedTime());
-    if (++nExtraNonce >= 0x7f && nNow > nPrevTime+15)
+    static uint256 hashPrevBlock;
+    if (hashPrevBlock != pblock->hashPrevBlock)
     {
-        nExtraNonce = 1;
-        nPrevTime = nNow;
+        nExtraNonce = 0;
+        hashPrevBlock = pblock->hashPrevBlock;
     }
+    ++nExtraNonce;
     pblock->vtx[0].vin[0].scriptSig = CScript() << pblock->nBits << CBigNum(nExtraNonce);
     pblock->hashMerkleRoot = pblock->BuildMerkleTree();
 }