diff --git a/include/amici/model.h b/include/amici/model.h
index d355ea7fee..481164afdf 100644
--- a/include/amici/model.h
+++ b/include/amici/model.h
@@ -704,6 +704,12 @@ class Model : public AbstractModel, public ModelDimensions {
 
     /**
      * @brief Set simulation start time.
+     *
+     * Output timepoints are absolute timepoints, independent of
+     * \f$ t_{0} \f$.
+     * For output timepoints \f$ t <  t_{0} \f$, the initial state will be
+     * returned.
+
      * @param t0 Simulation start time
      */
     void setT0(double t0);
diff --git a/include/amici/simulation_parameters.h b/include/amici/simulation_parameters.h
index 55db090184..8a1c1ec23d 100644
--- a/include/amici/simulation_parameters.h
+++ b/include/amici/simulation_parameters.h
@@ -175,7 +175,14 @@ class SimulationParameters {
      */
     std::vector<int> plist;
 
-    /** starting time */
+    /**
+     * @brief Starting time of the simulation.
+     *
+     * Output timepoints are absolute timepoints, independent of
+     * \f$ t_{start} \f$.
+     * For output timepoints \f$ t <  t_{start} \f$, the initial state will be
+     * returned.
+     */
     realtype tstart_{0.0};
 
     /**