diff --git a/evmcore/dummy_block.go b/evmcore/dummy_block.go
index 314b4de6..4314413f 100644
--- a/evmcore/dummy_block.go
+++ b/evmcore/dummy_block.go
@@ -17,7 +17,6 @@
 package evmcore
 
 import (
-	"encoding/binary"
 	"math"
 	"math/big"
 	"time"
@@ -135,9 +134,6 @@ func (h *EvmHeader) EthHeader() *types.Header {
 	if h == nil {
 		return nil
 	}
-	extra := make([]byte, 16)
-	binary.BigEndian.PutUint64(extra[:8], uint64(h.Time))
-	binary.BigEndian.PutUint64(extra[8:], uint64(h.Duration))
 	// NOTE: incomplete conversion
 	ethHeader := &types.Header{
 		Number:     h.Number,
@@ -148,7 +144,7 @@ func (h *EvmHeader) EthHeader() *types.Header {
 		TxHash:     h.TxHash,
 		ParentHash: h.ParentHash,
 		Time:       uint64(h.Time.Unix()),
-		Extra:      extra,
+		Extra:      inter.EncodeExtraData(h.Time.Time(), h.Duration),
 		BaseFee:    h.BaseFee,
 
 		Difficulty: new(big.Int),
@@ -195,7 +191,6 @@ type EvmBlockJson struct {
 }
 
 func (h *EvmHeader) ToJson(receipts types.Receipts) *EvmHeaderJson {
-	ethHeader := h.EthHeader()
 	enc := &EvmHeaderJson{
 		Number:          (*hexutil.Big)(h.Number),
 		Miner:           h.Coinbase,
@@ -207,7 +202,7 @@ func (h *EvmHeader) ToJson(receipts types.Receipts) *EvmHeaderJson {
 		UncleHash:       types.EmptyUncleHash,
 		Time:            hexutil.Uint64(h.Time.Unix()),
 		TimeNano:        hexutil.Uint64(h.Time),
-		Extra:           ethHeader.Extra,
+		Extra:           inter.EncodeExtraData(h.Time.Time(), h.Duration),
 		BaseFee:         (*hexutil.Big)(h.BaseFee),
 		Difficulty:      new(hexutil.Big),
 		PrevRandao:      h.PrevRandao,
diff --git a/gossip/apply_genesis.go b/gossip/apply_genesis.go
index ae7baa8c..863abb19 100644
--- a/gossip/apply_genesis.go
+++ b/gossip/apply_genesis.go
@@ -77,14 +77,11 @@ func (s *Store) ApplyGenesis(g genesis.Genesis) (err error) {
 		if rules.Upgrades.Sonic {
 			block := s.GetBlock(br.Idx - 1)
 			if block == nil {
-				block = &inter.Block{
-					BaseFee: big.NewInt(0),
-				}
+				block = &inter.Block{BaseFee: new(big.Int)}
 			}
 			header := &evmcore.EvmHeader{
-				GasUsed:  block.GasUsed,
-				GasLimit: block.GasLimit,
-				BaseFee:  block.BaseFee,
+				GasUsed: block.GasUsed,
+				BaseFee: block.BaseFee,
 			}
 			baseFee = gasprice.GetBaseFeeForNextBlock(header, rules.Economy)
 			duration = time.Duration(br.Time-block.Time) * time.Nanosecond
diff --git a/gossip/c_block_callbacks.go b/gossip/c_block_callbacks.go
index 831af91c..add9d865 100644
--- a/gossip/c_block_callbacks.go
+++ b/gossip/c_block_callbacks.go
@@ -2,13 +2,14 @@ package gossip
 
 import (
 	"fmt"
-	"github.com/Fantom-foundation/go-opera/utils/signers/gsignercache"
 	"math/big"
 	"sort"
 	"sync"
 	"sync/atomic"
 	"time"
 
+	"github.com/Fantom-foundation/go-opera/utils/signers/gsignercache"
+
 	"github.com/Fantom-foundation/lachesis-base/hash"
 	"github.com/Fantom-foundation/lachesis-base/inter/dag"
 	"github.com/Fantom-foundation/lachesis-base/inter/idx"
@@ -381,17 +382,13 @@ func consensusCallbackBeginBlockFn(
 						feed.newLogs.Send(logs)
 					}
 
-					lastBlockTime := evmStateReader.GetHeader(common.Hash{}, uint64(blockCtx.Idx-1)).Time.Time()
-					thisBlockTime := block.Time.Time()
-					blockTime := thisBlockTime.Sub(lastBlockTime)
-
 					now := time.Now()
 					blockAge := now.Sub(block.Time.Time())
 					log.Info("New block",
 						"index", blockCtx.Idx,
 						"id", block.Hash(),
 						"gas_used", evmBlock.GasUsed,
-						"gas_rate", float64(evmBlock.GasUsed)/blockTime.Seconds(),
+						"gas_rate", float64(evmBlock.GasUsed)/blockDuration.Seconds(),
 						"base_fee", evmBlock.BaseFee.String(),
 						"txs", fmt.Sprintf("%d/%d", len(evmBlock.Transactions), len(skippedTxs)),
 						"age", utils.PrettyDuration(blockAge),
diff --git a/inter/block.go b/inter/block.go
index d6e8e548..6e5c007f 100644
--- a/inter/block.go
+++ b/inter/block.go
@@ -2,6 +2,7 @@ package inter
 
 import (
 	"encoding/binary"
+	"errors"
 	"math/big"
 	"slices"
 	"time"
@@ -63,10 +64,6 @@ func (b *Block) Hash() common.Hash {
 
 // GetEthereumHeader returns the Ethereum header corresponding to this block.
 func (b *Block) GetEthereumHeader() *types.Header {
-	// TODO: consider condensing the extra data into a single 8-byte field.
-	extra := make([]byte, 16)
-	binary.BigEndian.PutUint64(extra[:8], uint64(b.Time))     // < only nano-second part needed
-	binary.BigEndian.PutUint64(extra[8:], uint64(b.Duration)) // < could be measured in microseconds instead of nanoseconds
 	return &types.Header{
 		ParentHash:  b.ParentHash,
 		UncleHash:   types.EmptyUncleHash,
@@ -80,10 +77,13 @@ func (b *Block) GetEthereumHeader() *types.Header {
 		GasLimit:    b.GasLimit,
 		GasUsed:     b.GasUsed,
 		Time:        uint64(b.Time.Time().Unix()),
-		Extra:       extra,
-		MixDigest:   b.PrevRandao,
-		Nonce:       types.BlockNonce{}, // constant 0 in Ethereum
-		BaseFee:     b.BaseFee,
+		Extra: EncodeExtraData(
+			b.Time.Time(),
+			time.Duration(b.Duration)*time.Nanosecond,
+		),
+		MixDigest: b.PrevRandao,
+		Nonce:     types.BlockNonce{}, // constant 0 in Ethereum
+		BaseFee:   b.BaseFee,
 
 		// Sonic does not have a beacon chain and no withdrawals.
 		WithdrawalsHash: &types.EmptyWithdrawalsHash,
@@ -95,6 +95,37 @@ func (b *Block) GetEthereumHeader() *types.Header {
 	}
 }
 
+// EncodeExtraData produces the ExtraData field encoding Sonic-specific data
+// in the Ethereum block header. This data includes:
+//   - the nano-second part of the block's timestamp, for sub-second precision;
+//   - the duration of the block, in nanoseconds, defined as the time elapsed
+//     between the predecessor block's timestamp and this block's timestamp.
+//     This is used for the computation of gas rates to adjust the base fee.
+func EncodeExtraData(time time.Time, duration time.Duration) []byte {
+	if duration < 0 {
+		duration = 0
+	}
+	extra := make([]byte, 12)
+	binary.BigEndian.PutUint32(extra[:4], uint32(time.Nanosecond()))
+	binary.BigEndian.PutUint64(extra[4:], uint64(duration.Nanoseconds()))
+	return extra
+}
+
+// DecodeExtraData decodes the ExtraData field encoding Sonic-specific data
+// in the Ethereum block header. See EncodeExtraData for details.
+func DecodeExtraData(extra []byte) (
+	nanos int,
+	duration time.Duration,
+	err error,
+) {
+	if len(extra) != 12 {
+		return 0, 0, errors.New("extra data must be 12 bytes long")
+	}
+	return int(binary.BigEndian.Uint32(extra[:4])),
+		time.Duration(binary.BigEndian.Uint64(extra[4:])),
+		nil
+}
+
 func (b *Block) EstimateSize() int {
 	return int(unsafe.Sizeof(*b)) +
 		len(b.TransactionHashes)*int(unsafe.Sizeof(common.Hash{}))
diff --git a/tests/block_header_test.go b/tests/block_header_test.go
index 0eddb87c..15b79212 100644
--- a/tests/block_header_test.go
+++ b/tests/block_header_test.go
@@ -2,13 +2,13 @@ package tests
 
 import (
 	"context"
-	"encoding/binary"
 	"math/big"
 	"testing"
 	"time"
 
 	"github.com/Fantom-foundation/go-opera/evmcore"
 	"github.com/Fantom-foundation/go-opera/gossip/gasprice"
+	"github.com/Fantom-foundation/go-opera/inter"
 	"github.com/Fantom-foundation/go-opera/opera"
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/core/types"
@@ -109,13 +109,22 @@ func testHeaders_GasUsedIsBelowGasLimit(t *testing.T, headers []*types.Header) {
 
 func testHeaders_EncodesDurationAndNanoTimeInExtraData(t *testing.T, headers []*types.Header) {
 	require := require.New(t)
+
+	getUnixTime := func(header *types.Header) time.Time {
+		t.Helper()
+		nanos, _, err := inter.DecodeExtraData(header.Extra)
+		require.NoError(err)
+		return time.Unix(int64(header.Time), int64(nanos))
+	}
+
 	// Check the nano-time and duration encoded in the extra data field.
 	for i := 1; i < len(headers); i++ {
-		require.Equal(len(headers[i].Extra), 16, "extra data length of block %d", i)
-		lastTime := binary.BigEndian.Uint64(headers[i-1].Extra[:8])
-		currentTime := binary.BigEndian.Uint64(headers[i].Extra[:8])
-		wantedDuration := currentTime - lastTime
-		gotDuration := binary.BigEndian.Uint64(headers[i].Extra[8:])
+		require.Equal(len(headers[i].Extra), 12, "extra data length of block %d", i)
+		lastTime := getUnixTime(headers[i-1])
+		currentTime := getUnixTime(headers[i])
+		wantedDuration := currentTime.Sub(lastTime)
+		_, gotDuration, err := inter.DecodeExtraData(headers[i].Extra)
+		require.NoError(err, "decoding extra data of block %d", i)
 		require.Equal(wantedDuration, gotDuration, "duration of block %d", i)
 	}
 }
@@ -132,13 +141,12 @@ func testHeaders_BaseFeeEvolutionFollowsPricingRules(t *testing.T, headers []*ty
 
 	// All other blocks compute the base-fee based on the previous block.
 	for i := 1; i < len(headers); i++ {
+		_, duration, err := inter.DecodeExtraData(headers[i-1].Extra)
+		require.NoError(err, "decoding extra data of block %d", i-1)
 		last := &evmcore.EvmHeader{
 			BaseFee:  headers[i-1].BaseFee,
-			GasLimit: headers[i-1].GasLimit,
 			GasUsed:  headers[i-1].GasUsed,
-			Duration: time.Duration(
-				binary.BigEndian.Uint64(headers[i-1].Extra[8:]),
-			),
+			Duration: duration,
 		}
 		require.Equal(
 			gasprice.GetBaseFeeForNextBlock(last, rules),