datetime_from_now

This is a quick Python script to accept a datetime, apply a delta, and display the result

Why?

The initial reason for putting this tool together was to assist in finding AWS EC2 instances older than a certain age. Here's a result that came up in search results:

aws/aws-cli#1209

The short version is to use the AWS CLI like so:

aws ec2 describe-instances --query 'Reservations[].Instances[?LaunchTime>=`2015-03-01`][].{id: InstanceId, type: InstanceType, launched: LaunchTime}'

The original page includes some shell scripting to get around different invocations of GNU and BSD 'date' commands. To get around that ugliness, a script written in Python was written. Consider this:

aws ec2 describe-instances --query 'Reservations[].Instances[?LaunchTime>=$(from_now.py --utc --hours -1)][].{id: InstanceId, type: InstanceType, launched: LaunchTime}'

This matches instances launched in the past hour. Isn't that fun?!

Installation

This script only imports from datetime which is a part of the Python Standard Library; therefore, no extra modules are required. Similarly, the entire program is self-contained in a single file; it can be copied anywhere where Python is installed and run there.

Usage

The following is the result of using the --help flag:

usage: from_now.py [-h] [-w WEEKS] [-d DAYS] [-H HOURS] [-m MINUTES]
                   [-s SECONDS] [-i INPUT_FORMAT] [-o OUTPUT_FORMAT] [-u]
                   [date]

This tool will find the date / time relative to another date / time.To use
this tool, provide a date/time (the default is the current date / time) and
the number of weeks, hours, minutes, and seconds from the provided date. The
tool will then write the resulting date/time to STDOUT.The relative values
(e.g., weeks, days, etc.) may be positive (meaning the resulting date will be
after the provided date) or negative (meaning the resulting date will be
before the provided date.A date / time format may also be specified with
--input-format and --output-format and defaults to ISO-8601

positional arguments:
  date                  date /time from which to calculate (defaults to
                        current date / time)

optional arguments:
  -h, --help            show this help message and exit
  -w WEEKS, --weeks WEEKS
                        number of weeks
  -d DAYS, --days DAYS  number of days
  -H HOURS, --hours HOURS
                        number of hours
  -m MINUTES, --minutes MINUTES
                        number of minutes
  -s SECONDS, --seconds SECONDS
                        number of seconds
  -i INPUT_FORMAT, --input-format INPUT_FORMAT
                        date input format string
  -o OUTPUT_FORMAT, --output-format OUTPUT_FORMAT
                        date output format string
  -u, --utc             set to use UTC; otherwise, use local timezone

Default format

The default time format is ISO-8601; this corresponds to the Python datetime format string of: %Y-%m-%dT%%H%M%S.%fZ

Examples

from_now.py --utc --days 7

Result: 2019-05-27T23:14:27.522175Z

This will return the date / time exactly seven days from right now.

from_now.py --hours -1 --output-format "%Y%m%d"

Result: 20190520

This will return the date / time exactly one hour ago.