Countdown timer in Pygame

Another easy way is to simply use pygame's event system.

Here's a simple example:

import pygame
pygame.init()
screen = pygame.display.set_mode((128, 128))
clock = pygame.time.Clock()

counter, text = 10, '10'.rjust(3)
pygame.time.set_timer(pygame.USEREVENT, 1000)
font = pygame.font.SysFont('Consolas', 30)

run = True
while run:
    for e in pygame.event.get():
        if e.type == pygame.USEREVENT: 
            counter -= 1
            text = str(counter).rjust(3) if counter > 0 else 'boom!'
        if e.type == pygame.QUIT: 
            run = False

    screen.fill((255, 255, 255))
    screen.blit(font.render(text, True, (0, 0, 0)), (32, 48))
    pygame.display.flip()
    clock.tick(60)

On this page you will find what you are looking for http://www.pygame.org/docs/ref/time.html#pygame.time.get_ticks
You download ticks once before beginning the countdown (which can be a trigger in the game - the key event, whatever). For example:

start_ticks=pygame.time.get_ticks() #starter tick
while mainloop: # mainloop
    seconds=(pygame.time.get_ticks()-start_ticks)/1000 #calculate how many seconds
    if seconds>10: # if more than 10 seconds close the game
        break
    print (seconds) #print how many seconds