internal/t8ntool: fix error condition

cmd/evm/internal/t8ntool/transition.go

@@ -259,8 +259,9 @@ func Main(ctx *cli.Context) error {
 			return NewError(ErrorVMConfig, errors.New("currentDifficulty was not provided, and cannot be calculated due to missing parentDifficulty"))
 		case env.Number == 0:
 			return NewError(ErrorVMConfig, errors.New("currentDifficulty needs to be provided for block number 0"))
-		case env.Timestamp == 0:
-			return NewError(ErrorVMConfig, errors.New("currentDifficulty cannot be calculated if time is set to 0"))
+		case env.Timestamp <= env.ParentTimestamp:
+			return NewError(ErrorVMConfig, fmt.Errorf("currentDifficulty cannot be calculated -- currentTime (%d) needs to be after parent time (%d)",
+				env.Timestamp, env.ParentTimestamp))
 		}
 		prestate.Env.Difficulty = calcDifficulty(chainConfig, env.Number, env.Timestamp,
 			env.ParentTimestamp, env.ParentDifficulty, env.ParentUncleHash)