import time


def unix_timestamp_to_string(unix_seconds, format_string):
	"""Converts a timestamp (in unix seconds since the epoch) to a human-readable string in UTC.
	format_string should be in a format that time.strftime understands.
	"""
	if (unix_seconds is None):
		return None
	return time.strftime(format_string, time.gmtime(unix_seconds))