72 lines
1.9 KiB
Markdown
72 lines
1.9 KiB
Markdown
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---
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id: l29u
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tags:
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- unix
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- time
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created: Sunday, April 28, 2024
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---
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# Time and computers
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## Epochs and Unix Time
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An epoch is a period of time identified by a starting point.
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The standard epoch for measuring against is known colloqially as "Unix Time":
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midnight on 1st January 1970. Unix time is the number of seconds that have
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elapsed since this point. This is the standard for Windows and most unix
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systems.
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For example midnight on 2nd January 1970 would be 86400 seconds since the epoch.
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The deduction is as follows:
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```
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60 * 60 * 24 = 86400
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```
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We are multiplpying the number of seconds in a minute by the number of minutes
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in an hour by the number of hours in a day.
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To represent times and dates before 1970, we use negative integers.
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## UTC and time zones
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"Coordinated Universal Time" (UTC) is the time standard against which the
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world's timekeeping is synchronized. It is not itself a timezone, it is a
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transcendent standard that defines what time zones are.
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Time zones are defined relative to UTC, by their offset to this value.
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For example North America is behind UTC by five or six hours hence uses the
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notation UTC-5:00 or UTC-6:00.
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UTC is the successor to GMT of which there were variances necessitating a
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universal value. Despite this GMT is equal to UTC+00:00 so apart from when
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daylight saving is in effect in the UK, GMT = UTC.
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Depending on your locale, relative to UTC, you will get different outputs when
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you attempt to log Unix Time.
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Also daylight savings can impact on the outputs. For instance for half of the
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year, when DST applies, the UK is UTC+01:00.
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## Examples in Python
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The following gives us the unix seconds in my current time zone (GMT DST):
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```py
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import time
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print(time.time())
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# 1714322393.2929392
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```
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If I want UTC, I can do:
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```py
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import datetime
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print(datetime.datetime.utcnow().strftime('%s'))
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# '1714318952'
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```
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We see clearly that they are not identical.
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