diff --git a/core/block_processor.go b/core/block_processor.go
index e912c0b6e1..f50ebb55a2 100644
--- a/core/block_processor.go
+++ b/core/block_processor.go
@@ -386,7 +386,7 @@ func ValidateHeader(pow pow.PoW, block *types.Header, parent *types.Block, check
 		return BlockEqualTSErr
 	}
 
-	expd := CalcDifficulty(int64(block.Time), int64(parent.Time()), parent.Difficulty())
+	expd := CalcDifficulty(block.Time, parent.Time(), parent.Difficulty())
 	if expd.Cmp(block.Difficulty) != 0 {
 		return fmt.Errorf("Difficulty check failed for block %v, %v", block.Difficulty, expd)
 	}
diff --git a/core/chain_makers.go b/core/chain_makers.go
index 07670608a5..501fe7a921 100644
--- a/core/chain_makers.go
+++ b/core/chain_makers.go
@@ -171,7 +171,7 @@ func makeHeader(parent *types.Block, state *state.StateDB) *types.Header {
 		Root:       state.Root(),
 		ParentHash: parent.Hash(),
 		Coinbase:   parent.Coinbase(),
-		Difficulty: CalcDifficulty(int64(time), int64(parent.Time()), parent.Difficulty()),
+		Difficulty: CalcDifficulty(time, parent.Time(), parent.Difficulty()),
 		GasLimit:   CalcGasLimit(parent),
 		GasUsed:    new(big.Int),
 		Number:     new(big.Int).Add(parent.Number(), common.Big1),
diff --git a/core/chain_manager.go b/core/chain_manager.go
index bd49bafc21..3c5eb0e8a9 100644
--- a/core/chain_manager.go
+++ b/core/chain_manager.go
@@ -611,7 +611,7 @@ func (self *ChainManager) InsertChain(chain types.Blocks) (int, error) {
 				// Allow up to MaxFuture second in the future blocks. If this limit
 				// is exceeded the chain is discarded and processed at a later time
 				// if given.
-				if max := time.Now().Unix() + maxTimeFutureBlocks; int64(block.Time()) > max {
+				if max := uint64(time.Now().Unix()) + maxTimeFutureBlocks; block.Time() > max {
 					return i, fmt.Errorf("%v: BlockFutureErr, %v > %v", BlockFutureErr, block.Time(), max)
 				}
 
diff --git a/core/chain_util.go b/core/chain_util.go
index 96c9a03d8f..7e3d8eba8e 100644
--- a/core/chain_util.go
+++ b/core/chain_util.go
@@ -31,10 +31,16 @@ import (
 // CalcDifficulty is the difficulty adjustment algorithm. It returns
 // the difficulty that a new block b should have when created at time
 // given the parent block's time and difficulty.
-func CalcDifficulty(time int64, parentTime int64, parentDiff *big.Int) *big.Int {
+func CalcDifficulty(time, parentTime uint64, parentDiff *big.Int) *big.Int {
 	diff := new(big.Int)
 	adjust := new(big.Int).Div(parentDiff, params.DifficultyBoundDivisor)
-	if big.NewInt(time-parentTime).Cmp(params.DurationLimit) < 0 {
+	bigTime := new(big.Int)
+	bigParentTime := new(big.Int)
+
+	bigTime.SetUint64(time)
+	bigParentTime.SetUint64(parentTime)
+
+	if bigTime.Sub(bigTime, bigParentTime).Cmp(params.DurationLimit) < 0 {
 		diff.Add(parentDiff, adjust)
 	} else {
 		diff.Sub(parentDiff, adjust)
diff --git a/miner/worker.go b/miner/worker.go
index 79514b2311..0857217514 100644
--- a/miner/worker.go
+++ b/miner/worker.go
@@ -427,7 +427,7 @@ func (self *worker) commitNewWork() {
 	header := &types.Header{
 		ParentHash: parent.Hash(),
 		Number:     num.Add(num, common.Big1),
-		Difficulty: core.CalcDifficulty(int64(tstamp), int64(parent.Time()), parent.Difficulty()),
+		Difficulty: core.CalcDifficulty(uint64(tstamp), parent.Time(), parent.Difficulty()),
 		GasLimit:   core.CalcGasLimit(parent),
 		GasUsed:    new(big.Int),
 		Coinbase:   self.coinbase,