I gradually provide several solutions with the most robust solution at the very end of this answer that tries to handle the following issues:
- utc offset due to DST
- past dates when the local timezone might have had different utc offset due to reason unrelated to DST.
dateutil
and stdlib solutions fail here on some systems, notably Windows
- ambiguous times during DST (don't know whether
Arrow
provides interface to handle it)
- non-existent times during DST (the same)
To find POSIX timestamp for tomorrow's midnight (or other fixed hour) in a given timezone, you could use code from How do I get the UTC time of “midnight” for a given timezone?:
from datetime import datetime, time, timedelta
import pytz
DAY = timedelta(1)
tz = pytz.timezone('America/Montreal')
tomorrow = datetime(2013, 11, 3).date() + DAY
midnight = tz.localize(datetime.combine(tomorrow, time(0, 0)), is_dst=None)
timestamp = (midnight - datetime(1970, 1, 1, tzinfo=pytz.utc)).total_seconds()
dt.date()
method returns the same naive date for both naive and timezone-aware dt
objects.
The explicit formula for timestamp is used to support Python version before Python 3.3. Otherwise .timestamp()
method could be used in Python 3.3+.
To avoid ambiguity in parsing input dates during DST transitions that are unavoidable for .localize()
method unless you know is_dst
parameter, you could use Unix timestamps stored with the dates:
from datetime import datetime, time, timedelta
import pytz
DAY = timedelta(1)
tz = pytz.timezone('America/Montreal')
local_dt = datetime.fromtimestamp(timestamp_from_the_log, tz)
tomorrow = local_dt.date() + DAY
midnight = tz.localize(datetime.combine(tomorrow, time(0, 0)), is_dst=None)
timestamp = (midnight - datetime(1970, 1, 1, tzinfo=pytz.utc)).total_seconds()
To support other fixed hours (not only midnight):
tomorrow = local_dt.replace(tzinfo=None) + DAY # tomorrow, same time
dt_plus_day = tz.localize(tomorrow, is_dst=None)
timestamp = dt_plus_day.timestamp() # use the explicit formula before Python 3.3
is_dst=None
raises an exception if the result date is ambiguous or non-existent. To avoid exception, you could choose the time that is closest to the previous date from yesterday (same DST state i.e., is_dst=local_dt.dst()
):
from datetime import datetime, time, timedelta
import pytz
DAY = timedelta(1)
tz = pytz.timezone('America/Montreal')
local_dt = datetime.fromtimestamp(timestamp_from_the_log, tz)
tomorrow = local_dt.replace(tzinfo=None) + DAY
dt_plus_day = tz.localize(tomorrow, is_dst=local_dt.dst())
dt_plus_day = tz.normalize(dt_plus_day) # to detect non-existent times
timestamp = (dt_plus_day - datetime(1970, 1, 1, tzinfo=pytz.utc)).total_seconds()
.localize()
respects given time even if it is non-existent, therefore .normalize()
is required to fix the time. You could raise an exception here if normalize()
method changes its input (non-existent time detected in this case) for consistency with other code examples.