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I have an UNIX Timestamp in my Arduino Sketch and would like to get the day of the week of this timestamp (1-7).

How can I convert the timestamp to get the day of the week?

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Do you mean, your program will be given a UNIX Timestamp number as data and should produce a day-of-the-week number? Or that you have a timestamp number and want to know its day-of-the-week number? If the latter, the Unix date command can tell you, via format code +%u. Eg, if date +%s says 1455407637 (for Sat Feb 13 16:53:57 MST 2016), date -d@1455407637 +%u says 6. – jwpat7 Feb 13 '16 at 23:58
up vote 1 down vote

The simplest solution to that problem is to download the Time library and use the weekday() function:

int weekday(time_t t); // the weekday for the given time

Cheers!

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One caveat, the breakTime() code at line 168 of Time.cpp computes Wday = (t/(60*60*24) + 4) % 7) + 1, ie, it apparently ignores leap seconds [26 in number since 1972] so for 26 seconds before every midnight the day-of-the-week calc will be off by 1. – jwpat7 Feb 14 '16 at 0:12
2  
Ignore previous comment. As noted in wikipedia's Unix time article, Unix timestamps don't count leap seconds, so they are locally correct and calculations near midnight will be ok, except during a leap second itself. Also see Eric Raymond's Time, Clock, and Calendar Programming In C article, and the article Unix leap seconds. – jwpat7 Feb 14 '16 at 0:58
    
UNIX timestamps are also kept in GMT/UTC time. So you may run into problems with it being some hours out. If so look to the functions gmtime() Vs locatime(). You may need to set your time-zone. Oh and watch out for daylight savings time if it matters for your project. – Kingsley Feb 15 '16 at 3:30

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