Use Calcite terminology

core/src/main/java/org/apache/calcite/sql/fun/SqlLibraryOperators.java

@@ -956,10 +956,10 @@ private SqlLibraryOperators() {
               OperandTypes.TIMESTAMP, OperandTypes.timestampInterval()),
           SqlFunctionCategory.TIMEDATE);
 
-  /** The "DATETIME_TRUNC(datetime_expression, date_time_part)"
-   * function (BigQuery); truncates a DATETIME value to the granularity of
-   * date_time_part. The DATETIME value is always rounded to the beginning of
-   * date_time_part. */
+  /** The "DATETIME_TRUNC(timestamp, timeUnit)"
+   * function (BigQuery); truncates a timestamp value to the granularity of
+   * timeUnit. The timestamp value is always rounded to the beginning of
+   * timeUnit. */
   @LibraryOperator(libraries = {BIG_QUERY})
   public static final SqlFunction DATETIME_TRUNC =
       SqlBasicFunction.create(

site/_docs/reference.md

@@ -2655,7 +2655,7 @@ BigQuery's type system uses confusingly different names for types and functions:
 | b | DATETIME_ADD(timestamp, interval)              | Returns the TIMESTAMP value that occurs *interval* after *timestamp*
 | b | DATETIME_DIFF(timestamp, timestamp2, timeUnit) | Returns the whole number of *timeUnit* between *timestamp* and *timestamp2*
 | b | DATETIME_SUB(timestamp, interval)              | Returns the TIMESTAMP that occurs *interval* before *timestamp*
-| b | DATETIME_TRUNC(datetimeExpression, timeUnit)   | Truncates *datetimeExpression* to the granularity of *timeUnit*, rounding to the beginning of the unit
+| b | DATETIME_TRUNC(timestamp, timeUnit)            | Truncates *timestamp* to the granularity of *timeUnit*, rounding to the beginning of the unit
 | b | DATE_FROM_UNIX_DATE(integer)                   | Returns the DATE that is *integer* days after 1970-01-01
 | p | DATE_PART(timeUnit, datetime)                  | Equivalent to `EXTRACT(timeUnit FROM  datetime)`
 | b | DATE_ADD(date, interval)                       | Returns the DATE value that occurs *interval* after *date*