ClickHouse/doc/reference/en/functions/date_time_functions.rst

Functions for working with dates and times
--------------------------------------

Time Zone Support

All functions for working with the date and time for which this makes sense, can take a second, optional argument - the time zone name. Example: Asia / Yekaterinburg. In this case, they do not use the local time zone (the default), but the specified one.
.. code-block:: sql
  SELECT
      toDateTime('2016-06-15 23:00:00') AS time,
      toDate(time) AS date_local,
      toDate(time, 'Asia/Yekaterinburg') AS date_yekat,
      toString(time, 'US/Samoa') AS time_samoa
  
  ┌────────────────time─┬─date_local─┬─date_yekat─┬─time_samoa──────────┐
  │ 2016-06-15 23:00:00 │ 2016-06-15 │ 2016-06-16 │ 2016-06-15 09:00:00 │
  └─────────────────────┴────────────┴────────────┴─────────────────────┘
Only time zones are supported, different from UTC for an integer number of hours.

toYear
~~~~~~~
Converts a date or date with time to a UInt16 number containing the year number (AD).

toMonth
~~~~~~~
Converts a date or date with time to a UInt8 number containing the month number (1-12).

toDayOfMonth
~~~~~~~
Converts a date or date with time to a UInt8 number containing the number of the day of the month (1-31).

toDayOfWeek
~~~~~~~
Converts a date or date with time to a UInt8 number containing the number of the day of the week (Monday is 1, and Sunday is 7).

toHour
~~~~~~~
Converts a date with time to a UInt8 number containing the number of the hour in 24-hour time (0-23).
This function assumes that if clocks are moved ahead, it is by one hour and occurs at 2 a.m., and if clocks are moved back, it is by one hour and occurs at 3 a.m. (which is not always true - even in Moscow the clocks were once changed at a different time).

toMinute
~~~~~~~
Converts a date with time to a UInt8 number containing the number of the minute of the hour (0-59).

toSecond
~~~~~~~
Converts a date with time to a UInt8 number containing the number of the second in the minute (0-59).
Leap seconds are not accounted for.

toStartOfDay
~~~~~~~
Rounds down a date with time to the start of the day.

toMonday
~~~~~~~
Rounds down a date or date with time to the nearest Monday.
Returns the date.

toStartOfMonth
~~~~~~~
Rounds down a date or date with time to the first day of the month.
Returns the date.

toStartOfQuarter
~~~~~~~
Rounds down a date or date with time to the first day of the quarter.
The first day of the quarter is either 1 January, 1 April, 1 July, or 1 October. Returns the date.

toStartOfYear
~~~~~~~
Rounds down a date or date with time to the first day of the year.
Returns the date.

toStartOfMinute
~~~~~~~
Rounds down a date with time to the start of the minute.

toStartOfFiveMinute
~~~~~~~
Rounds down a date with time to the start of the 5 minute (00:00, 00:05, 00:10...).

toStartOfHour
~~~~~~~
Rounds down a date with time to the start of the hour.

toTime
~~~~~~~
Converts a date with time to some fixed date, while preserving the time.

toRelativeYearNum
~~~~~~~
Converts a date with time or date to the number of the year, starting from a certain fixed point in the past.

toRelativeMonthNum
~~~~~~~
Converts a date with time or date to the number of the month, starting from a certain fixed point in the past.

toRelativeWeekNum
~~~~~~~
Converts a date with time or date to the number of the week, starting from a certain fixed point in the past.

toRelativeDayNum
~~~~~~~
Converts a date with time or date to the number of the day, starting from a certain fixed point in the past.

toRelativeHourNum
~~~~~~~
Converts a date with time or date to the number of the hour, starting from a certain fixed point in the past.

toRelativeMinuteNum
~~~~~~~
Converts a date with time or date to the number of the minute, starting from a certain fixed point in the past.

toRelativeSecondNum
~~~~~~~
Converts a date with time or date to the number of the second, starting from a certain fixed point in the past.

now
~~~~~~~
Accepts zero arguments and returns the current time at one of the moments of request execution.
This function returns a constant, even if the request took a long time to complete.

today
~~~~~~~
Accepts zero arguments and returns the current date at one of the moments of request execution.
The same as 'toDate(now())'.

yesterday
~~~~~~~
Accepts zero arguments and returns yesterday's date at one of the moments of request execution.
The same as 'today() - 1'.

timeSlot
~~~~~~~
Rounds the time to the half hour.
This function is specific to Yandex.Metrica, since half an hour is the minimum amount of time for breaking a session into two sessions if a counter shows a single user's consecutive pageviews that differ in time by strictly more than this amount. This means that tuples (the counter number, user ID, and time slot) can be used to search for pageviews that are included in the corresponding session.

timeSlots(StartTime, Duration)
~~~~~~~
For a time interval starting at 'StartTime' and continuing for 'Duration' seconds, it returns an array of moments in time, consisting of points from this interval rounded down to the half hour.
For example, timeSlots(toDateTime('2012-01-01 12:20:00'), toUInt32(600)) = [toDateTime('2012-01-01 12:00:00'), toDateTime('2012-01-01 12:30:00')].
This is necessary for searching for pageviews in the corresponding session.