fix the comments for time transformer

Author: WZhuo

src/iceberg/transform.h

@@ -111,25 +111,25 @@ class ICEBERG_EXPORT Transform : public util::Formattable {
 
   /// \brief Creates a shared singleton instance of the Year transform.
   ///
-  /// Extracts the year portion from a date or timestamp.
+  /// Extracts the number of years from a date or timestamp since the epoch.
   /// \return A shared pointer to the Year transform.
   static std::shared_ptr<Transform> Year();
 
   /// \brief Creates a shared singleton instance of the Month transform.
   ///
-  /// Extracts the month portion from a date or timestamp.
+  /// Extracts the number of months from a date or timestamp since the epoch.
   /// \return A shared pointer to the Month transform.
   static std::shared_ptr<Transform> Month();
 
   /// \brief Creates a shared singleton instance of the Day transform.
   ///
-  /// Extracts the day portion from a date or timestamp.
+  /// Extracts the number of days from a date or timestamp since the epoch.
   /// \return A shared pointer to the Day transform.
   static std::shared_ptr<Transform> Day();
 
   /// \brief Creates a shared singleton instance of the Hour transform.
   ///
-  /// Extracts the hour portion from a timestamp.
+  /// Extracts the number of hours from a timestamp since the epoch.
   /// \return A shared pointer to the Hour transform.
   static std::shared_ptr<Transform> Hour();
 

src/iceberg/transform_function.h

@@ -100,13 +100,14 @@ class ICEBERG_EXPORT TruncateTransform : public TransformFunction {
   int32_t width_;
 };
 
-/// \brief Year transform that extracts the year component from timestamp inputs.
+/// \brief Year transform that extracts the number of years from timestamp inputs since
+/// the epoch.
 class ICEBERG_EXPORT YearTransform : public TransformFunction {
  public:
   /// \param source_type Must be a timestamp type.
   explicit YearTransform(std::shared_ptr<Type> const& source_type);
 
-  /// \brief Extract a date or timestamp year, as years from 1970.
+  /// \brief Extract the number of years since the epoch.
   Result<Literal> Transform(const Literal& literal) override;
 
   /// \brief Returns INT32 as the output type.
@@ -119,13 +120,14 @@ class ICEBERG_EXPORT YearTransform : public TransformFunction {
       std::shared_ptr<Type> const& source_type);
 };
 
-/// \brief Month transform that extracts the month component from timestamp inputs.
+/// \brief Month transform that extracts the number of months from timestamp inputs since
+/// the epoch.
 class ICEBERG_EXPORT MonthTransform : public TransformFunction {
  public:
   /// \param source_type Must be a timestamp type.
   explicit MonthTransform(std::shared_ptr<Type> const& source_type);
 
-  /// \brief Extract a date or timestamp month, as months from 1970-01-01.
+  /// \brief Extract the number of months since the epoch.
   Result<Literal> Transform(const Literal& literal) override;
 
   /// \brief Returns INT32 as the output type.
@@ -138,13 +140,14 @@ class ICEBERG_EXPORT MonthTransform : public TransformFunction {
       std::shared_ptr<Type> const& source_type);
 };
 
-/// \brief Day transform that extracts the day of the month from timestamp inputs.
+/// \brief Day transform that extracts the number of days from timestamp inputs since the
+/// epoch.
 class ICEBERG_EXPORT DayTransform : public TransformFunction {
  public:
   /// \param source_type Must be a timestamp type.
   explicit DayTransform(std::shared_ptr<Type> const& source_type);
 
-  /// \brief Extract a date or timestamp day, as days from 1970-01-01.
+  /// \brief Extract the number of days since the epoch.
   Result<Literal> Transform(const Literal& literal) override;
 
   /// \brief Return the result type of a day transform.
@@ -161,13 +164,14 @@ class ICEBERG_EXPORT DayTransform : public TransformFunction {
       std::shared_ptr<Type> const& source_type);
 };
 
-/// \brief Hour transform that extracts the hour component from timestamp inputs.
+/// \brief Hour transform that extracts the number of hours from timestamp inputs since
+/// the epoch.
 class ICEBERG_EXPORT HourTransform : public TransformFunction {
  public:
   /// \param source_type Must be a timestamp type.
   explicit HourTransform(std::shared_ptr<Type> const& source_type);
 
-  /// \brief Extract a timestamp hour, as hours from 1970-01-01 00:00:00.
+  /// \brief Extract the number of hours since the epoch.
   Result<Literal> Transform(const Literal& literal) override;
 
   /// \brief Returns INT32 as the output type.