fix FillMissingBatches edge case, and correct spec off by 1

op-node/rollup/derive/batches.go

@@ -70,8 +70,8 @@ func FillMissingBatches(batches []*BatchData, epoch eth.BlockID, blockTime, minL
 	// - fill up to the next L1 block timestamp, if higher, to keep up with L1 time
 	// - fill up to the last valid batch, to keep up with L2 time
 	newHeadL2Timestamp := minL2Time
-	if nextL1Time > newHeadL2Timestamp+blockTime {
-		newHeadL2Timestamp = nextL1Time - blockTime
+	if nextL1Time > newHeadL2Timestamp+1 {
+		newHeadL2Timestamp = nextL1Time - 1
 	}
 	for _, b := range batches {
 		m[b.BatchV1.Timestamp] = b

op-node/rollup/derive/l1_block_info_test.go

@@ -6,7 +6,6 @@ import (
 	"testing"
 
 	"github.com/ethereum-optimism/optimism/op-node/eth"
-
 	"github.com/ethereum-optimism/optimism/op-node/testutils"
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/stretchr/testify/assert"

specs/rollup-node.md

@@ -181,8 +181,8 @@ To encode user-deposited transactions, refer to the following sections of the de
 A sequencing window is derived into a variable number of L2 blocks, defined by a range of timestamps:
 
 - Starting at `min_l2_timestamp`, as defined in the batch filtering.
-- Up to and including
-  `new_head_l2_timestamp = max(highest_valid_batch_timestamp, next_l1_timestamp - l2_block_time, min_l2_timestamp)`
+- Up to and including (including only if aligned with L2 block time)
+  `new_head_l2_timestamp = max(highest_valid_batch_timestamp, next_l1_timestamp - 1, min_l2_timestamp)`
   - `highest_valid_batch_timestamp = max(batch.timestamp for batch in filtered_batches)`,
     or `0` if no there are no `filtered_batches`.
     `batch.timestamp` refers to the L2 block timestamp encoded in the batch.