Develop Reference

Converting local time to Zulu time in order to compare times

I have a DB with time entries formatted as follow:
2018-11-05T08:58:00Z
I'm trying to generate SQL queries to compare "now()" with the time in the DB to determine which row(s) to return.
I'm battling to "convert" my local time (now()) to an equivalent time format so that I can use < or > operations against the DB values.
Additionally, I am not sure if the problem has two parts. The example fo the time above is not in a "Datetime" field in MySQL but stored simply as TEXT, leaving me to suspect that I would need to "convert" the DB entries into another format first?

The following code, using the datetime module, works for me (tested in Python 3.6):
import datetime
value = "2018-11-05T08:58:00Z"
dt = datetime.datetime.strptime(value, "%Y-%m-%dT%H:%M:%SZ")
# Result is: datetime.datetime(2018, 11, 5, 8, 58)
This will convert your string values to datetime instances, which you can then compare to now(). The values that get created should be naive (meaning they have no associated timezone information).
However, if you are sure that now() for you is not UTC (aka Zulu time), you may need to do a conversion. This could be possible if, for example, you are using Django's timezone.now() and your configured timezone is something other than UTC. In this case, I might convert the result of now() to UTC, so you only have to convert one value. The pytz module can easily handle this kind of thing.

Check this :
import time
time = time.strftime('%Y-%m-%dT%H:%M:%SZ')
print(time)

Datetime returns wrong time in unix

I have a wired situation here with returning the time in unix format.
I have this function:
def get_today_date():
nowDate = str(datetime.datetime.utcnow().date())
unix_date = time.mktime(datetime.datetime.strptime(nowDate, "%Y-%m-%d").timetuple())
return (unix_date)
it's straightforward when I print the value of "nowDate" -> it gives me the right value I want (ex: 7/29/2018 00:00:00 UTC) which is correct. But when I change the format from Unix timestamp to date format and get only the date without time, It gives me (7/28/2018).
I've took the value in unix and checked it, it gives me (7/28/2018 21:00:00). Why?
My laptop is in a UTC +3 timezone.
Did something go wrong in the conversion? Or its an internal error in my laptop makes the output wrong?

If you're looking for today's date/time, use datetime.datetime.now(), if you need just the date, use datetime.datetime.today() it's automatically converted into your timezone.
If you play with UTC-based functions, you'll get 3 hours offset for your location, which sometimes might result in time being 21:00 of previous day instead of midnight of the next one -- that's quite reasonable =)
you may use the following approach:
utcnow = datetime.datetime.utcnow()
midnight = time.mktime( datetime.datetime( utcnow.year, utcnow.month, utcnow.day).timetuple() )

Convert timestamp to postgres timestamp

Currently i have the following timestamp format as below :
2018-04-02T09:00:00+09:30
How can i convert the timestamp above to suit the postgres's timestamp column like below?
2018-04-02 09:00:00 +09.30
Also how can use python to convert the xml timestamp first before loading into postgres table?

This doesn't require any conversion whatsoever, it's a regular ISO 8601 time stamp. PostgreSQL supports multiple input formats for time stamps
select timestamp with time zone '2018-04-02T09:00:00+09:30';
timestamptz
------------------------
2018-04-01 18:30:00-05
(1 row)
You'll notice it's storing it in UTC. That's what you want. From the docs,
For timestamp with time zone, the internally stored value is always in UTC (Universal Coordinated Time, traditionally known as Greenwich Mean Time, GMT). An input value that has an explicit time zone specified is converted to UTC using the appropriate offset for that time zone. If no time zone is stated in the input string, then it is assumed to be in the time zone indicated by the system's TimeZone parameter, and is converted to UTC using the offset for the timezone zone.
When a timestamp with time zone value is output, it is always converted from UTC to the current timezone zone, and displayed as local time in that zone. To see the time in another time zone, either change timezone or use the AT TIME ZONE construct (see Section 9.9.3).

If you know you will always get your data in that format, you can do something like:
>>> a = "2018-04-02T09:00:00+09:30"
>>>
>>> b = a.replace('T', ' ').replace('+', ' +')
>>> b
'2018-04-02 09:00:00 +09:30'
It is ugly, but it works.
It is always safer to interpret your input as a date and then print it in the desired format.

Python: creating list of timestamps by minute

I am trying to figure out what the best way to create a list of timestamps in Python is, where the values for the items in the list increment by one minute. The timestamps would be by minute, and would be for the previous 24 hours. I need to create timestamps of the format "MM/dd/yyy HH:mm:ss" or to at least contain all of those measures. The timestamps will be an axis for a graph of data that I am collecting.
Calculating the times alone isn't too bad, as I could just get the current time, convert it to seconds, and change the value by one minute very easily. However, I am kind of stuck on figuring out the date aspect of it without having to do a lot of checking, which doesn't feel very Pythonic.
Is there an easier way to do this? For example, in JavaScript, you can get a Date() object, and simply subtract one minute from the value and JS will take care of figuring out if any of the other fields need to change and how they need to change.

datetime is the way to go, you might want to check out This Blog.
import datetime
import time
now = datetime.datetime.now()
print now
print now.ctime()
print now.isoformat()
print now.strftime("%Y%m%dT%H%M%S")
This would output
2003-08-05 21:36:11.590000
Tue Aug 5 21:36:11 2003
2003-08-05T21:36:11.590000
20030805T213611
You can also do subtraction with datetime and timedelta objects
now = datetime.datetime.now()
minute = timedelta(days=0,seconds=60,microseconds=0)
print now-minute
would output
2015-07-06 10:12:02.349574

You are looking for datetime and timedelta objects. See the docs.

Difference between Python datetime vs time modules

I am trying to figure out the differences between the datetime and time modules, and what each should be used for.
I know that datetime provides both dates and time. What is the use of the time module?
Examples would be appreciated and differences concerning timezones would especially be of interest.

The time module is principally for working with Unix time stamps; expressed as a floating point number taken to be seconds since the Unix epoch. the datetime module can support many of the same operations, but provides a more object oriented set of types, and also has some limited support for time zones.

Stick to time to prevent DST ambiguity.
Use exclusively the system time module instead of the datetime module to prevent ambiguity issues with daylight savings time (DST).
Conversion to any time format, including local time, is pretty easy:
import time
t = time.time()
time.strftime('%Y-%m-%d %H:%M %Z', time.localtime(t))
'2019-05-27 12:03 CEST'
time.strftime('%Y-%m-%d %H:%M %Z', time.gmtime(t))
'2019-05-27 10:03 GMT'
time.time() is a floating point number representing the time in seconds since the system epoch. time.time() is ideal for unambiguous time stamping.
If the system additionally runs the network time protocol (NTP) dæmon, one ends up with a pretty solid time base.
Here is the documentation of the time module.

The time module can be used when you just need the time of a particular record - like lets say you have a seperate table/file for the transactions for each day, then you would just need the time.
However the time datatype is usually used to store the time difference between 2 points of time.
This can also be done using datetime, but if we are only dealing with time for a particular day, then time module can be used.
Datetime is used to store a particular data and time for a record. Like in a rental agency. The due date would be a datetime datatype.

Just noticed that time is more precise than datetime with an extra digit.
import time as tm
from datetime import datetime as dt
restime = tm.time()
resdtime = dt.timestamp(dt.now())
print("TIME:".rjust(10," "),restime)
print("DATETIME:".rjust(10," "),resdtime)
Output
TIME: 1637357103.7650678
DATETIME: 1637357103.765067