Is there a way to extract the moment of historic leap seconds from the time-zone database that is distributed on most linux distributions? I am looking for a solution in python, but anything that works on the command line would be fine too.
My use case is to convert between gps-time (which is basically the number of seconds since the first GPS-satellite was switched on in 1980) and UTC or local time. UTC is adjusted for leap-seconds every now and then, while gps-time increases linearly. This is equivalent to converting between UTC and TAI. TAI also ignores leap-seconds, so TAI and gps-time should always evolve with the same offset. At work, we use gps-time as the time standard for synchronizing astronomical observations around the world.
I have working functions that convert between gps-time and UTC, but I had to hard-code a table of leap seconds, which I get here (the file tzdata2013xx.tar.gz contains a file named leapseconds). I have to update this file by hand every few years when a new leapsecond is announced. I would prefer to get this information from the standard tzdata, which is automatically updated via system updates several times a year.
I am pretty sure the information is hidden in some binary files somewhere in /usr/share/zoneinfo/. I have been able to extract some of it using struct.unpack (man tzfile gives some info about the format), but I never got it working completely. Are there any standard packages that can access this information? I know about pytz, which seems to get the standard DST information from the same database, but it does not give access to leap seconds. I also found tai64n, but looking at its source code, it just contains a hard-coded table.
EDIT
Inspired by steveha's answer and some code in pytz/tzfile.py, I finally got a working solution (tested on py2.5 and py2.7):
from struct import unpack, calcsize
from datetime import datetime
def print_leap(tzfile = '/usr/share/zoneinfo/right/UTC'):
with open(tzfile, 'rb') as f:
# read header
fmt = '>4s c 15x 6l'
(magic, format, ttisgmtcnt, ttisstdcnt,leapcnt, timecnt,
typecnt, charcnt) = unpack(fmt, f.read(calcsize(fmt)))
assert magic == 'TZif'.encode('US-ASCII'), 'Not a timezone file'
print 'Found %i leapseconds:' % leapcnt
# skip over some uninteresting data
fmt = '>%(timecnt)dl %(timecnt)dB %(ttinfo)s %(charcnt)ds' % dict(
timecnt=timecnt, ttinfo='lBB'*typecnt, charcnt=charcnt)
f.read(calcsize(fmt))
#read leap-seconds
fmt = '>2l'
for i in xrange(leapcnt):
tleap, nleap = unpack(fmt, f.read(calcsize(fmt)))
print datetime.utcfromtimestamp(tleap-nleap+1)
with result
In [2]: print_leap()
Found 25 leapseconds:
1972-07-01 00:00:00
1973-01-01 00:00:00
1974-01-01 00:00:00
...
2006-01-01 00:00:00
2009-01-01 00:00:00
2012-07-01 00:00:00
While this does solve my question, I will probably not go for this solution. Instead, I will include leap-seconds.list with my code, as suggested by Matt Johnson. This seems to be the authoritative list used as a source for tzdata, and is probably updated by NIST twice a year. This means I will have to do the update by hand, but this file is straightforward to parse and includes an expiration date (which tzdata seems to be missing).
I just did man 5 tzfile and computed an offset that would find the leap seconds info, then read the leap seconds info.
You can uncomment the "DEBUG:" print statements to see more of what it finds in the file.
EDIT: program updated to now be correct. It now uses the file /usr/share/zoneinfo/right/UTC and it now finds leap-seconds to print.
The original program wasn't skipping the timezeone abbreviation characters, which are documented in the man page but sort of hidden ("...and tt_abbrind serves as an index into the array of timezone abbreviation characters that follow the ttinfo structure(s) in the file.").
import datetime
import struct
TZFILE_MAGIC = 'TZif'.encode('US-ASCII')
def leap_seconds(f):
"""
Return a list of tuples of this format: (timestamp, number_of_seconds)
timestamp: a 32-bit timestamp, seconds since the UNIX epoch
number_of_seconds: how many leap-seconds occur at timestamp
"""
fmt = ">4s c 15x 6l"
size = struct.calcsize(fmt)
(tzfile_magic, tzfile_format, ttisgmtcnt, ttisstdcnt, leapcnt, timecnt,
typecnt, charcnt) = struct.unpack(fmt, f.read(size))
#print("DEBUG: tzfile_magic: {} tzfile_format: {} ttisgmtcnt: {} ttisstdcnt: {} leapcnt: {} timecnt: {} typecnt: {} charcnt: {}".format(tzfile_magic, tzfile_format, ttisgmtcnt, ttisstdcnt, leapcnt, timecnt, typecnt, charcnt))
# Make sure it is a tzfile(5) file
assert tzfile_magic == TZFILE_MAGIC, (
"Not a tzfile; file magic was: '{}'".format(tzfile_magic))
# comments below show struct codes such as "l" for 32-bit long integer
offset = (timecnt*4 # transition times, each "l"
+ timecnt*1 # indices tying transition time to ttinfo values, each "B"
+ typecnt*6 # ttinfo structs, each stored as "lBB"
+ charcnt*1) # timezone abbreviation chars, each "c"
f.seek(offset, 1) # seek offset bytes from current position
fmt = '>{}l'.format(leapcnt*2)
#print("DEBUG: leapcnt: {} fmt: '{}'".format(leapcnt, fmt))
size = struct.calcsize(fmt)
data = struct.unpack(fmt, f.read(size))
lst = [(data[i], data[i+1]) for i in range(0, len(data), 2)]
assert all(lst[i][0] < lst[i+1][0] for i in range(len(lst)-1))
assert all(lst[i][1] == lst[i+1][1]-1 for i in range(len(lst)-1))
return lst
def print_leaps(leap_lst):
# leap_lst is tuples: (timestamp, num_leap_seconds)
for ts, num_secs in leap_lst:
print(datetime.datetime.utcfromtimestamp(ts - num_secs+1))
if __name__ == '__main__':
import os
zoneinfo_fname = '/usr/share/zoneinfo/right/UTC'
with open(zoneinfo_fname, 'rb') as f:
leap_lst = leap_seconds(f)
print_leaps(leap_lst)
来源：https://stackoverflow.com/questions/19332902/extract-historic-leap-seconds-from-tzdata