Try using jump_timer = dbTimer() and then doing a subtraction in your conditions code e.g see below code taken from a mario clone platform game:
if (dbTimer() - jumpStartTimer < 650)
playerY = playerY - speedy;
if (dbTimer() - jumpStartTimer > 720)
jumping = false;
The variable "jumping" is just a boolean variable to test if the character is jumping or not e.g. if the jump key has been pressed and if the character is touching the ground.
In your example code, I couldn't see where you are testing to see if the user has pressed the key for jumping etc....