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Subtracting datetime in string format with datetime format

I have 2 variables.
One is datetime in string format and the other is datetime in datetime.datetime format.
For example -
2021-09-06T07:58:19.032Z # string
2021-09-05 14:58:10.209675 # datetime.datetime
I want to find out the difference between these 2 times in seconds.
I think we need to have both in datetime before we can do this subtraction.
I'm having a hard time converting the string to datetime.
Can someone please help.

You can convert the string into datetime object with strptime()
An example with your given dates:
from datetime import datetime
# Assuming this is already a datetime object in your code, you don't need this part
# I needed this part to be able to use it as a datetime object
date1 = datetime.strptime("2021-09-05 14:58:10.209675", "%Y-%m-%d %H:%M:%S.%f")
## The part where the string is converted to datetime object
# Since the string has "T" and "Z", we will have to remove them before we convert
formatted = "2021-09-06T07:58:19.032Z".replace("T", " ").replace("Z", "")
>>> 2021-09-06 07:58:19.032
# Finally, converting the string
date2 = datetime.strptime(formatted, "%Y-%m-%d %H:%M:%S.%f")
# Now date2 variable is a datetime object
# Performing a simple operation
print(date1 - date2)
>>> -1 day, 6:59:51.177675

Convert the str to datetime via strptime() and then get the difference of the 2 datetime objects in seconds via total_seconds().
from datetime import datetime, timezone
# Input
dt1_str = "2021-09-06T07:58:19.032Z" # String type
dt2 = datetime(year=2021, month=9, day=5, hour=14, minute=58, second=10, microsecond=209675, tzinfo=timezone.utc) # datetime type
# Convert the string to datetime
dt1 = datetime.strptime(dt1_str, "%Y-%m-%dT%H:%M:%S.%f%z")
# Subtract the datetime objects and get the seconds
diff_seconds = (dt1 - dt2).total_seconds()
print(diff_seconds)
Output
61208.822325

The first string time you mention could be rfc3339 format.
A module called python-dateutil could help
import dateutil.parser
dateutil.parser.parse('2021-09-06T07:58:19.032Z')
datetime module could parse this time format by
datetime.datetime.strptime("2021-09-06T07:58:19.032Z","%Y-%m-%dT%H:%M:%S.%fZ")
But this way may cause trouble when get a time in another timezone because it doesn't support timezone offset.

Datetime formatting

The api I am working with gives time is the following format when I place an order.
'orderDateTime': '12-May-2020 14:54:11'
What I am looking to do is to find the number of minutes/seconds that have passed since I placed the order. So if it has already been for example 10 minutes, if I would like to cancel or modify the order I can do it.
I have tried everything I know to convert the given time format to do what I want but have been unsuccessful. Please help. Thanks in advance.
time_now = datetime.now()
print("Time now is",time_now)
t1 = time_now.strftime("%d-%b-%Y %H:%M:%S")
print(t1)
trade_time = datetime(12-May-2020 15:01:32)
t2 = datetime.strftime(trade_time,"%d-%b-%Y %H:%M:%S")
print(t2)

You are mixing the datetime objects and their representation as strings.
What you need to do is convert your trade time to a datetime object, by using the strptime method. strftime does the opposite, it produces a formatted text representation of your datetime.
Then, you can subtract the two datetime objects, which will give you the difference as a timedelta, from which you can get the difference as a number of seconds.
So, your code should look like:
from datetime import datetime
time_now = datetime.now()
trade_time_as_str = '12-May-2020 15:01:32'
trade_time = datetime.strptime(trade_time_as_str,"%d-%b-%Y %H:%M:%S")
elapsed = time_now - trade_time
elapsed_seconds = elapsed.total_seconds()
print(elapsed_seconds)
# 15808.77104

How to combine dates and times?

I want to add a time to a datetime. My initial datetime is: initial_datetime='2015-11-03 08:05:22' and is a string and this_hour and this_min are strings too. I use:
time='-7:00'
time = time.split(':')
this_hour = time[0]
this_min = time[1]
initial_datetime='2015-11-03 08:05:22'
new_date = datetime.combine(initial_datetime, time(this_hour, this_min))
+ timedelta(hours=4)
But there comes an error:
'str' object is not callable.
My desired output is the initial_datetime plus my time (in this case -7 hours ) and then add 4 hours. So, in my example, the new date should be '2015-11-03 05:05:22'.

datetime.combine is typically used to combine a date object with a time object rather than incrementing or decrementing a datetime object. In your case, you need to convert your datetime string to a datetime object and convert the parts of your time string to integers so you can add them to your datetime with timedelta. As an aside, be careful about using variable names, like time, that conflict with your imports.
from datetime import datetime, timedelta
dtstr = '2015-11-03 08:05:22'
tstr = '-7:00'
hours, minutes = [int(t) for t in tstr.split(':')]
dt = datetime.strptime(dtstr, '%Y-%m-%d %H:%M:%S') + timedelta(hours=hours+4, minutes=minutes)
print(dt)
# 2015-11-03 05:05:22

converting a list of string with the format 68.830320 in 'time' format without date information. The time format is second.millisecond

I have a string in the form of 68.830320 Format. this I need to convert to time Format in second.millisecond. it does not contain date or any other values. I cannot use strptime since the Format is not right. tstamp that I'm trying to parse is a list of calues containg values with decimal Point. I cannot round this value. it still gives error. I'm not sure how to proceeed. please help!
tried a lot of threads from here that always take the datetime object. But since my Format is not in the same way, I cannot use that info. I have tries .time dateutil, and everything else available. I still cannot solve this problem
tstamp = child2.get('timestamp').replace(" ", "").replace("\n", "")
print(tstamp)
parser.parser(tstamp)
format_time = datetime.date(tstamp)
print(format_time)

A number of seconds isn't a datetime, it's a timedelta. It isn't a datetime because you can't take the string "68.830320" and set the hands on a wall clock to represent that time.
Convert your string to a timedelta like this:
>>> from datetime import timedelta
>>> mytime = timedelta(seconds=float("68.830320"))
>>> mytime
datetime.timedelta(0, 68, 830320)
You can then add the timedelta to a datetime to get a wall clock time.

How to get UTC time in Python?

How do I get the UTC time, i.e. milliseconds since Unix epoch on Jan 1, 1970?

For Python 2 code, use datetime.utcnow():
from datetime import datetime
datetime.utcnow()
For Python 3, use datetime.now(timezone.utc) (the 2.x solution will technically work, but has a giant warning in the 3.x docs):
from datetime import datetime, timezone
datetime.now(timezone.utc)
For your purposes when you need to calculate an amount of time spent between two dates all that you need is to subtract end and start dates. The results of such subtraction is a timedelta object.
From the python docs:
class datetime.timedelta([days[, seconds[, microseconds[, milliseconds[, minutes[, hours[, weeks]]]]]]])
And this means that by default you can get any of the fields mentioned in it's definition -
days, seconds, microseconds, milliseconds, minutes, hours, weeks. Also timedelta instance has total_seconds() method that:
Return the total number of seconds contained in the duration.
Equivalent to (td.microseconds + (td.seconds + td.days * 24 * 3600) *
106) / 106 computed with true division enabled.

Timezone-aware datetime object, unlike datetime.utcnow():
from datetime import datetime,timezone
now_utc = datetime.now(timezone.utc)
Timestamp in milliseconds since Unix epoch:
datetime.now(timezone.utc).timestamp() * 1000

In the form closest to your original:
import datetime
def UtcNow():
now = datetime.datetime.utcnow()
return now
If you need to know the number of seconds from 1970-01-01 rather than a native Python datetime, use this instead:
return (now - datetime.datetime(1970, 1, 1)).total_seconds()
Python has naming conventions that are at odds with what you might be used to in Javascript, see PEP 8. Also, a function that simply returns the result of another function is rather silly; if it's just a matter of making it more accessible, you can create another name for a function by simply assigning it. The first example above could be replaced with:
utc_now = datetime.datetime.utcnow

import datetime
import pytz
# datetime object with timezone awareness:
datetime.datetime.now(tz=pytz.utc)
# seconds from epoch:
datetime.datetime.now(tz=pytz.utc).timestamp()
# ms from epoch:
int(datetime.datetime.now(tz=pytz.utc).timestamp() * 1000)

Timezone aware with zero external dependencies:
from datetime import datetime, timezone
def utc_now():
return datetime.utcnow().replace(tzinfo=timezone.utc)

From datetime.datetime you already can export to timestamps with method strftime. Following your function example:
import datetime
def UtcNow():
now = datetime.datetime.utcnow()
return int(now.strftime("%s"))
If you want microseconds, you need to change the export string and cast to float like: return float(now.strftime("%s.%f"))

you could use datetime library to get UTC time even local time.
import datetime
utc_time = datetime.datetime.utcnow()
print(utc_time.strftime('%Y%m%d %H%M%S'))

why all reply based on datetime and not time?
i think is the easy way !
import time
nowgmt = time.strftime("%Y-%m-%d %H:%M:%S", time.gmtime())
print(nowgmt)

To be correct, UTC format needs at least the T letter:
>>> a=(datetime.datetime.now(timezone.utc))
>>> a.strftime("%Y-%m-%dT%H:%M:%SZ")
'2022-11-28T16:42:17Z'