Autosave: 2024-04-28 17:50:04

zk/Time_and_computers.md

@@ -5,7 +5,7 @@ tags: []
 created: Sunday, April 28, 2024
 ---
 
-# Time_and_computers
+# Time and computers
 
 ## Epochs and Unix Time
 
@@ -25,3 +25,46 @@ The deduction is as follows:
 
 We are multiplpying the number of seconds in a minute by the number of minutes
 in an hour by the number of hours in a day.
+
+To represent times and dates before 1970, we use negative integers.
+
+## UTC and time zones
+
+"Coordinated Universal Time" (UTC) is the time standard against which the
+world's timekeeping is synchronized. It is not itself a timezone, it is a
+transcendent standard that defines what time zones are.
+
+Time zones are defined relative to UTC, by their offset to this value.
+
+For example North America is behind UTC by five or six hours hence uses the
+notation UTC-5:00 or UTC-6:00.
+
+UTC is the successor to GMT of which there were variances necessitating a
+universal value. Despite this GMT is equal to UTC+00:00 so apart from when
+daylight saving is in effect in the UK, GMT = UTC.
+
+Depending on your locale, relative to UTC, you will get different outputs when
+you attempt to log Unix Time.
+
+Also daylight savings can impact on the outputs. For instance for half of the
+year, when DST applies, the UK is UTC+01:00.
+
+## Examples in Python
+
+The following gives us the unix seconds in my current time zone (GMT DST):
+
+```py
+import time
+print(time.time())
+# 1714322393.2929392
+```
+
+If I want UTC, I can do:
+
+```py
+import datetime
+print(datetime.datetime.utcnow().strftime('%s'))
+# '1714318952'
+```
+
+We see clearly that they are not identical.