Convert a Unix timestamp to a DATETIME in a View

The only truly safe formats for DATETIME/SMALLDATETIME in SQL Server are:

yyyyMMdd
yyyyMMdd hh:nn:ss[.mmmmmmm]
yyyy-MM-ddThh:nn:ss[.mmmmmmm]
----------^ yes, that T is important!

Anything else is subject to incorrect interpretation by SQL Server, Windows, the provider, the application code, end users, etc. For example, the following always breaks:*

SET LANGUAGE FRENCH;
SELECT CONVERT(DATETIME, '2013-11-13');

Result:

Le paramètre de langue est passé à Français.
Msg 242, Level 16, State 3, Line 2
La conversion d'un type de données varchar en type de données datetime a créé une valeur hors limites.

Just changing the language (which any of your user sessions can do) forced SQL Server to interpret that as YYYY-DD-MM instead of YYYY-MM-DD. Similar things can happen with setting like DATEFORMAT. But these settings are literally ignored when using the above two formats.

Always, always, always use one of the above two formats. If you are passing a variable as a string, stop doing that. If you can't, check to make sure it passes ISDATE() first. If you are letting people type any date string into a form field, stop doing that, too. Use a date-picker or calendar control and dictate the format of the string before you pass it to SQL Server. Well, depending on the language, just keep it as a datetime value and don't convert it to a string at all.

• I can't reproduce your problem at all, btw...

Please read this post:

• Bad Habits to Kick : Mis-handling date/range queries

_{There is an exception: SELECT CONVERT(DATE, 'yyyy-mm-dd'); will not break. But I err on the side of consistency rather than using a format only in the one place where I know it doesn't break, and having to use a safer format everywhere else.}

MySQL support a FROM_UNIXTIME(...) function :-)

select FROM_UNIXTIME(1433433155)

And the other way around, UNIX_TIMESTAMP

Applying it to your example, assuming timecreated and timemodified are those Unix timestamps:

SELECT
      category
      ,fullname
      ,shortname
      ,idnumber
      ,startdate
      ,visible
      , FROM_UNIXTIME (timecreated)
      , FROM_UNIXTIME (timemodified)
  ,count( my.id ) AS mydata
FROM mdl_mytable
JOIN mycontext ct ON ( ct.my = cat.id )
LEFT JOIN myjoin ra ON ( mycontext = cat.id )
WHERE ct.mynumber =10000000
GROUP BY shortname, fullname, .category
      ,.idnumber
      ,.startdate
      ,.visible
      , FROM_UNIXTIME (timecreated)
      , FROM_UNIXTIME (timemodified)
ORDER BY myorder ASC