How to convert a Unix timestamp to a civil date

Here is how to convert a Unix timestamp to a Gregorian (western) civil date and back in Ruby:

# Convert a Unix timestamp to a civil date ([year, month, day, hour, minute,
# second]) in GMT.
def timestamp_to_gmt_civil(ts)
  t = Time.at(ts).utc
  [t.year, t.month, t.mday, t.hour, t.min, t.sec]
end

# Convert a civil date in GMT to a Unix timestamp.
def gmt_civil_to_timestamp(y, m, d, h, mi, s)
  Time.utc(y, m, d, h, mi, s).to_i
end

Here is a pure algorithmic solution, which doesn't use the Time built-in class:

# Convert a Unix timestamp to a civil date ([year, month, day, hour, minute,
# second]) in GMT.
def timestamp_to_gmt_civil(ts)
  s = ts%86400
  ts /= 86400
  h = s/3600
  m = s/60%60
  s = s%60
  x = (ts*4+102032)/146097+15
  b = ts+2442113+x-(x/4)
  c = (b*20-2442)/7305
  d = b-365*c-c/4
  e = d*1000/30601
  f = d-e*30-e*601/1000
  (e < 14 ? [c-4716,e-1,f,h,m,s] : [c-4715,e-13,f,h,m,s])
end

# Convert a civil date in GMT to a Unix timestamp.
def gmt_civil_to_timestamp(y, m, d, h, mi, s)
  if m <= 2; y -= 1; m += 12; end
  (365*y + y/4 - y/100 + y/400 + 3*(m+1)/5 + 30*m + d - 719561) *
      86400 + 3600 * h + 60 * mi + s
end
t = Time.now.to_i
fail if t != gmt_civil_to_timestamp(*timestamp_to_gmt_civil(t))

Please note that most programming languages have a built-in function named gmtime for doing timestamp_to_gmt_civil's work. Please note that Python has calendar.timegm for doing gmt_civil_to_timestamp's work. Please note that most programming languages have a built-in function named mktime for converting a local time to a timestamp (compare with gmt_civil_to_timestamp, which accepts GMT time as input).

Please be aware of the rounding direction of division for negative numbers when porting the Ruby code above to other programming languages. For example, Ruby has 7 / (-4) == -2, but many other programming languages have 7 / (-4) == -1.

The algorithmic solution above is part of the programming folklore. See more examples at http://www.google.com/codesearch?q=2442+%2F\+*7305