logtimeshift.py

import sys
import re
import datetime

REGEX = "([0-9]{8} [0-9]{2}:[0-9]{2}:[0-9]{2}.[0-9]{3})(.*)";
shiftSeconds = int(sys.argv[2])
startTime = sys.argv[3]
startShifting = False

with open(sys.argv[1]) as fp:	
    for line in fp:
		m = re.search(REGEX, line)
		if m is not None:
			timestring = m.group(1)
			if timestring == startTime:
				startShifting = True

			if startShifting:	
				rest = m.group(2)        	      
				t = datetime.datetime.strptime(timestring, "%Y%m%d %H:%M:%S.%f")
				t = t - datetime.timedelta(seconds = shiftSeconds)
				shiftedTime = t.strftime("%Y%m%d %H:%M:%S.%f")[:-3]	
				print shiftedTime + rest			
			else:
				print m.group(0)