from threading import Timer
from datetime import datetime
x=2
def timenows():
    if x==1:
        now = datetime.now() # current date and time
        texttime = now.strftime("%H:%M:%S")
        timer = Timer(1, timenows)
        timer.start()
        print(texttime)
        return texttime
    else:
        if x==2:
            starttimeing = datetime.strptime('2017-05-04',"%Y-%m-%d")
            texttime = starttimeing.strftime("%H:%M:%S")
            timer = Timer(1, timenows)
            timer.start()
            print(texttime)
            return texttime
timenows()

  
>>> from datetime import datetime
>>> 
>>> starttimeing = datetime.strptime('2017-05-04',"%Y-%m-%d")
>>> texttime = starttimeing.strftime("%H:%M:%S")
>>> texttime
'00:00:00'
>>> starttimeing
datetime.datetime(2017, 5, 4, 0, 0)
>>>

 def timenows():
    while True:
        a = 0
        b = 0
        c = 0
        d = 0
        sec = 999 + 999*60 + 999*3600 + 999*216000
        for i in range(sec):
            time.sleep(1)
            a += 1
            #print('Прошло',a ,'Секунд',b,'минут', c, 'часов')
            print(c, 'часов', b,'минут',a ,'Секунд', d, 'мс')
            if a / 60 == 1:
                b += 1
                a = 0
            if b / 60 == 1:
                c += 1
                b = 0
timenows()

 import time 
def timenows():
    while True:
        z = 0
        a = 0
        b = 0
        c = 0
        d = 0
        sec = 999 + 999*60 + 999*3600 + 999*216000
        for i in range(sec):
            time.sleep(0.01)
            a += 1
            #print('Прошло',a ,'Секунд',b,'минут', c, 'часов')
            print(d, 'мс:', z, 'часов |', c, 'минут |', b,'секунд |',a ,'милисекунд' )
            if a / 60 == 1:
                b += 1
                a = 0
            if b / 60 == 1:
                c += 1
                b = 0
                d += 1
            if c / 60 == 1:
                d += 1
            
            d += 1
            timek = (d+ ' мс: '+ z + ' часов | '+ c +' минут |'+ b+' секунд |' + a + ' милисекунд' )
            return timek
timenows()