DST is probably the cause, but it could possibly be weirder things … timezones are strange. you can set the timezone to a variable, and then examine its properties to see its local offset (offset_std) and when that offset is valid from/until:

iex> tz = Timex.timezone("America/Chicago", {2015,4,12})
iex> tz.offset_std
3600
iex> z = Timex.timezone("America/Chicago", {2015,1,12})
iex> tz.offset_std
0
iex> tz.from 
{:sunday, {{2014, 11, 2}, {1, 0, 0}}}
iex> tz.until                                           
{:sunday, {{2015, 3, 8}, {2, 0, 0}}}