set time step when first loading meshlayer

python/core/auto_generated/mesh/qgsmeshdataprovidertemporalcapabilities.sip.in

@@ -101,6 +101,13 @@ Returns the relative time in milliseconds of the dataset
     void clear();
 %Docstring
 Clears alls stored reference times and dataset times
+%End
+
+    qint64 firstTimeStepDuration( int group ) const;
+%Docstring
+Returns the duration of the first time step of the dataset group with index ``group``
+
+The value is -1 if the dataset group is not present or if it contains only one dataset (non temporal dataset)
 %End
 
 };

python/core/auto_generated/mesh/qgsmeshlayer.sip.in

@@ -406,6 +406,13 @@ Doesn't take ownership of the pointed item, the root item is cloned.
 %Docstring
 Reset the dataset group tree item to default from provider
 
+.. versionadded:: 3.14
+%End
+
+    QgsInterval firstValidTimeStep() const;
+%Docstring
+Return the first valid time step of the dataset groups, invalid QgInterval if no time step is present
+
 .. versionadded:: 3.14
 %End
 

src/core/mesh/qgsmeshdataprovidertemporalcapabilities.cpp

@@ -174,3 +174,15 @@ void QgsMeshDataProviderTemporalCapabilities::clear()
   mGroupsReferenceDateTime.clear();
   mDatasetTimeSinceGroupReference.clear();
 }
+
+qint64 QgsMeshDataProviderTemporalCapabilities::firstTimeStepDuration( int group ) const
+{
+  qint64 ret = -1;
+  if ( mDatasetTimeSinceGroupReference.contains( group ) )
+  {
+    const QList<qint64> times = mDatasetTimeSinceGroupReference[group];
+    if ( times.count() > 1 )
+      ret = times.at( 1 ) - times.at( 0 );
+  }
+  return ret;
+}

src/core/mesh/qgsmeshdataprovidertemporalcapabilities.h

@@ -133,6 +133,13 @@ class CORE_EXPORT QgsMeshDataProviderTemporalCapabilities: public QgsDataProvide
     */
     void clear();
 
+    /**
+    * Returns the duration of the first time step of the dataset group with index \a group
+    *
+    * The value is -1 if the dataset group is not present or if it contains only one dataset (non temporal dataset)
+    */
+    qint64 firstTimeStepDuration( int group ) const;
+
   private:
 
     //! Holds the global reference date/time value if exists (min of all groups), otherwise it is invalid

src/core/mesh/qgsmeshlayer.cpp

@@ -735,6 +735,21 @@ void QgsMeshLayer::resetDatasetGroupTreeItem()
   updateActiveDatasetGroups();
 }
 
+QgsInterval QgsMeshLayer::firstValidTimeStep() const
+{
+  if ( !mDataProvider )
+    return QgsInterval();
+  int groupCount = mDataProvider->datasetGroupCount();
+  for ( int i = 0; i < groupCount; ++i )
+  {
+    qint64 timeStep = mDataProvider->temporalCapabilities()->firstTimeStepDuration( i );
+    if ( timeStep > 0 )
+      return QgsInterval( timeStep, QgsUnitTypes::TemporalMilliseconds );
+  }
+
+  return QgsInterval();
+}
+
 void QgsMeshLayer::updateActiveDatasetGroups()
 {
   if ( !mDatasetGroupTreeRootItem )

src/core/mesh/qgsmeshlayer.h

@@ -21,6 +21,7 @@
 #include <memory>
 
 #include "qgis_core.h"
+#include "qgsinterval.h"
 #include "qgsmaplayer.h"
 #include "qgsmeshdataprovider.h"
 #include "qgsmeshrenderersettings.h"
@@ -454,6 +455,13 @@ class CORE_EXPORT QgsMeshLayer : public QgsMapLayer
      */
     void resetDatasetGroupTreeItem();
 
+    /**
+     * Return the first valid time step of the dataset groups, invalid QgInterval if no time step is present
+     *
+     * \since QGIS 3.14
+     */
+    QgsInterval firstValidTimeStep() const;
+
   public slots:
 
     /**

src/gui/qgstemporalcontrollerwidget.cpp

@@ -24,6 +24,7 @@
 #include "qgstemporalmapsettingswidget.h"
 #include "qgstemporalutils.h"
 #include "qgsmaplayertemporalproperties.h"
+#include "qgsmeshlayer.h"
 
 #include <QAction>
 #include <QMenu>
@@ -389,19 +390,26 @@ void QgsTemporalControllerWidget::onLayersAdded( const QList<QgsMapLayer *> &lay
             if ( layer->isValid() && layer->temporalProperties()->isActive() && !mHasTemporalLayersLoaded )
             {
               mHasTemporalLayersLoaded = true;
-              // if we are moving from zero temporal layers to non-zero temporal layers, let's set temporal extent
-              this->setDatesToProjectTime();
+              firstTemporalLayerLoaded( layer );
             }
           } );
         }
+
+        firstTemporalLayerLoaded( layer );
       }
     }
-
-    if ( mHasTemporalLayersLoaded )
-      setDatesToProjectTime();
   }
 }
 
+void QgsTemporalControllerWidget::firstTemporalLayerLoaded( QgsMapLayer *layer )
+{
+  setDatesToProjectTime();
+
+  QgsMeshLayer *meshLayer = qobject_cast<QgsMeshLayer *>( layer );
+  if ( meshLayer )
+    setTimeStep( meshLayer->firstValidTimeStep() );
+}
+
 void QgsTemporalControllerWidget::onProjectCleared()
 {
   mHasTemporalLayersLoaded = false;
@@ -414,6 +422,8 @@ void QgsTemporalControllerWidget::onProjectCleared()
   whileBlocking( mFixedRangeStartDateTime )->setDateTime( QDateTime( QDate::currentDate(), QTime( 0, 0, 0, Qt::UTC ) ) );
   whileBlocking( mFixedRangeEndDateTime )->setDateTime( mStartDateTime->dateTime() );
   updateTemporalExtent();
+  mTimeStepsComboBox->setCurrentIndex( mTimeStepsComboBox->findData( QgsUnitTypes::TemporalHours ) );
+  mStepSpinBox->setValue( 1 );
 }
 
 void QgsTemporalControllerWidget::updateSlider( const QgsDateTimeRange &range )
@@ -498,6 +508,34 @@ void QgsTemporalControllerWidget::mRangeSetToAllLayersAction_triggered()
   saveRangeToProject();
 }
 
+void QgsTemporalControllerWidget::setTimeStep( const QgsInterval &timeStep )
+{
+  if ( ! timeStep.isValid() || timeStep.seconds() <= 0 )
+    return;
+  int indexUnit = -1;
+  double value = std::numeric_limits<double>::min();
+  double previousValue;
+  do
+  {
+    indexUnit++;
+    previousValue = value;
+    QgsUnitTypes::TemporalUnit unit = static_cast<QgsUnitTypes::TemporalUnit>( mTimeStepsComboBox->itemData( indexUnit ).toInt() );
+    value = timeStep.seconds() * QgsUnitTypes::fromUnitToUnitFactor( QgsUnitTypes::TemporalSeconds, unit );
+  }
+  while ( value > 10 ||  indexUnit == mTimeStepsComboBox->count() );
+
+  if ( fabs( log( value ) ) > fabs( log( previousValue ) ) )
+  {
+    indexUnit--;
+    value = previousValue;
+  }
+
+  mTimeStepsComboBox->setCurrentIndex( indexUnit );
+  mStepSpinBox->setValue( value );
+
+  updateFrameDuration();
+}
+
 void QgsTemporalControllerWidget::mRangeSetToProjectAction_triggered()
 {
   setDatesToProjectTime();

src/gui/qgstemporalcontrollerwidget.h

@@ -89,6 +89,9 @@ class GUI_EXPORT QgsTemporalControllerWidget : public QgsPanelWidget, private Ui
     std::unique_ptr< QMenu > mRangeLayersSubMenu;
     QgsMapLayerModel *mMapLayerModel = nullptr;
 
+    void firstTemporalLayerLoaded( QgsMapLayer *layer );
+    void setTimeStep( const QgsInterval &timeStep );
+
   private slots:
 
     /**