(3D) How can I make the player run faster if the player jumps again in 0.5 seconds after jumping?

Question

As the title says, what I'm trying to achieve is put a timer for the player. If the player jumps in less than half a second after the first jump I want to boost the player's speed by 5 if not reset the speed to 70. You may realize most of the code is actually from FPS tutorial in Godot tutorials. I just started to learn Godot and immediately tried to implement my own things. And as you can see I got stuck on this.

extends KinematicBody

const GRAVITY = -200.8
var vel = Vector3()
var maxSpeed = 70
var jumpSpeed = 80
const ACCEL = 70.0

var dir = Vector3()

const DEACCEL= 40
const MAX_SLOPE_ANGLE = 40

var camera
var rotation_helper

var MOUSE_SENSITIVITY = 1.0

var jumpTimer

func _ready():
    camera = $Rotation_Helper/Camera
    rotation_helper = $Rotation_Helper

    jumpTimer = $JumpTimer

    Input.set_mouse_mode(Input.MOUSE_MODE_CAPTURED)

func _physics_process(delta):
    process_input(delta)
    process_movement(delta)

func process_input(delta):

    # ----------------------------------
    # Walking
    dir = Vector3()
    var cam_xform = camera.get_global_transform()

    var input_movement_vector = Vector2()

    if Input.is_action_pressed("movement_forward"):
        input_movement_vector.y += 1
    if Input.is_action_pressed("movement_backward"):
        input_movement_vector.y -= 1
    if Input.is_action_pressed("movement_left"):
        input_movement_vector.x -= 1
    if Input.is_action_pressed("movement_right"):
        input_movement_vector.x += 1

    input_movement_vector = input_movement_vector.normalized()

    # Basis vectors are already normalized.
    dir += -cam_xform.basis.z * input_movement_vector.y
    dir += cam_xform.basis.x * input_movement_vector.x
    # ----------------------------------

This is the important part:

    # ----------------------------------
    # Jumping
    if is_on_floor():
        if Input.is_action_just_pressed("movement_jump"):
            vel.y = jumpSpeed
            print_debug(jumpTimer.time_left)
            if jumpTimer.time_left > 0.0:
                if maxSpeed < 100:
                    maxSpeed += 5

                jumpTimer.stop()

            else:
                maxSpeed = 70
                jumpTimer.start(0.5)

# ----------------------------------

    # ----------------------------------
    # Capturing/Freeing the cursor
    if Input.is_action_just_pressed("ui_cancel"):
        if Input.get_mouse_mode() == Input.MOUSE_MODE_VISIBLE:
            Input.set_mouse_mode(Input.MOUSE_MODE_CAPTURED)
        else:
            Input.set_mouse_mode(Input.MOUSE_MODE_VISIBLE)
    # ----------------------------------

func process_movement(delta):
    dir.y = 0
    dir = dir.normalized()

    vel.y += delta * GRAVITY

    var hvel = vel
    hvel.y = 0

    var target = dir
    target *= maxSpeed

    var accel
    if dir.dot(hvel) > 0:
        accel = ACCEL
    else:
        accel = DEACCEL

    hvel = hvel.linear_interpolate(target, accel * delta)
    vel.x = hvel.x
    vel.z = hvel.z
    vel = move_and_slide(vel, Vector3(0, 1, 0), 0.05, 4, deg2rad(MAX_SLOPE_ANGLE))

func _input(event):
    if event is InputEventMouseMotion and Input.get_mouse_mode() == Input.MOUSE_MODE_CAPTURED:
        rotation_helper.rotate_x(deg2rad(event.relative.y * MOUSE_SENSITIVITY))
        self.rotate_y(deg2rad(event.relative.x * MOUSE_SENSITIVITY * -1))

        var camera_rot = rotation_helper.rotation_degrees
        camera_rot.x = clamp(camera_rot.x, -70, 70)
        rotation_helper.rotation_degrees = camera_rot