diff --git a/opm/parser/eclipse/EclipseState/Schedule/TimeMap.cpp b/opm/parser/eclipse/EclipseState/Schedule/TimeMap.cpp
index 368ea90bd..753335231 100644
--- a/opm/parser/eclipse/EclipseState/Schedule/TimeMap.cpp
+++ b/opm/parser/eclipse/EclipseState/Schedule/TimeMap.cpp
@@ -191,7 +191,7 @@ namespace Opm {
         }
     } 
 
-    double TimeMap::getTimeStepLength(int tStepIdx) const
+    double TimeMap::getTimeStepLength(size_t tStepIdx) const
     {
         assert(0 <= tStepIdx && tStepIdx < numTimesteps());
         const boost::posix_time::ptime &t1
@@ -203,7 +203,7 @@ namespace Opm {
         return static_cast<double>(deltaT.total_milliseconds())/1000.0;
     }
 
-    double TimeMap::getTimePassedUntil(int tStepIdx) const
+    double TimeMap::getTimePassedUntil(size_t tStepIdx) const
     {
         assert(0 <= tStepIdx && tStepIdx < numTimesteps());
         const boost::posix_time::ptime &t1
diff --git a/opm/parser/eclipse/EclipseState/Schedule/TimeMap.hpp b/opm/parser/eclipse/EclipseState/Schedule/TimeMap.hpp
index c126bb4be..df3ecb435 100644
--- a/opm/parser/eclipse/EclipseState/Schedule/TimeMap.hpp
+++ b/opm/parser/eclipse/EclipseState/Schedule/TimeMap.hpp
@@ -41,12 +41,12 @@ namespace Opm {
         int numTimesteps() const { return m_timeList.size() - 1; }
         double getTotalTime() const;
         /// Return the date and time where a given time step starts.
-        boost::posix_time::ptime getStartTime(int tStepIdx) const
+        boost::posix_time::ptime getStartTime(size_t tStepIdx) const
         { return m_timeList[tStepIdx]; }
         /// Return the period of time in seconds which passed between the start of the simulation and a given time step.
-        double getTimePassedUntil(int tStepIdx) const;
+        double getTimePassedUntil(size_t tStepIdx) const;
         /// Return the length of a given time step in seconds.
-        double getTimeStepLength(int tStepIdx) const;
+        double getTimeStepLength(size_t tStepIdx) const;
         static boost::posix_time::ptime timeFromEclipse(DeckRecordConstPtr dateRecord);
         static boost::posix_time::ptime timeFromEclipse(int day , const std::string& month, int year, const std::string& eclipseTimeString = "00:00:00.000");
         static boost::posix_time::time_duration dayTimeFromEclipse(const std::string& eclipseTimeString);