add comments explaining calculation for epoch duration

chain/index/gc.go

@@ -76,9 +76,16 @@ func (si *SqliteIndexer) gc(ctx context.Context) {
 	// -------------------------------------------------------------------------------------------------
 	// Also GC eth hashes
 
-	currHeadTime := time.Unix(int64(head.MinTimestamp()), 0)
+	// Calculate the retention duration based on the number of epochs to retain.
+	// retentionDuration represents the total duration (in nano seconds) for which data should be retained before considering it for garbage collection.
+	// graceDuration represents the additional duration (in nano seconds) to retain data after the retention duration.
+	// Since time.Duration expects a nanosecond value, we multiply the total seconds by time.Second to convert it to nanoseconds.
 	retentionDuration := time.Duration(si.gcRetentionEpochs*builtin.EpochDurationSeconds) * time.Second
-	totalRetentionDuration := retentionDuration + (time.Duration(graceEpochs) * time.Duration(builtin.EpochDurationSeconds) * time.Second)
+	graceDuration := time.Duration(graceEpochs*builtin.EpochDurationSeconds) * time.Second
+
+	// Calculate the total duration to retain data.
+	totalRetentionDuration := retentionDuration + graceDuration
+	currHeadTime := time.Unix(int64(head.MinTimestamp()), 0)
 	// gcTime is the time that is (gcRetentionEpochs + graceEpochs) before currHeadTime
 	gcTime := currHeadTime.Add(-totalRetentionDuration)