Added documentation for date_trunc function.

docs/en/sql-reference/functions/date-time-functions.md

@@ -339,6 +339,22 @@ SELECT toDate('2016-12-27') AS date, toYearWeek(date) AS yearWeek0, toYearWeek(d
 └────────────┴───────────┴───────────┴───────────┘
 ```
 
+## date_trunc(datepart, time\_or\_data\[, time\_zone\]), dateTrunc(datepart, time\_or\_data\[, time\_zone\]) {#date_trunc}
+
+Truncates a date or date with time based on the specified datepart, such as
+- `second`
+- `minute`
+- `hour`
+- `day`
+- `week`
+- `month`
+- `quarter`
+- `year`
+
+```sql
+SELECT date_trunc('hour', now())
+```
+
 ## now {#now}
 
 Accepts zero arguments and returns the current time at one of the moments of request execution.