clarify behavior in off-normal cases

Author: gonuke

src/timer.cc

@@ -294,7 +294,15 @@ int Timer::dur() {
 int Timer::CalcTimeDiff(int year, int month) {
 
   int start_time = si_.y0 * cyclusYear + si_.m0 * cyclusMonth;
-  int time = year * cyclusYear + month * cyclusMonth;
+  int time = std::min(year,0) * cyclusYear + std::min(month,0) * cyclusMonth;
+
+  // if time is 0, then invalid combination of year and month were given
+  if time == 0 {
+    CLOG(LEV_WARN) << "Invalid year and month combination given to Timer::CalcTimeDiff. Returning 0. "
+                   "Year: " << year << " Month: " << month;
+
+    return 0;
+  } 
 
   // Casting because ctx_->dt() is uint64_t and so negatives don't play nice
   return (time - start_time) / static_cast<int>(ctx_->dt());

src/timer.h

@@ -75,14 +75,13 @@ class Timer {
   /// @return the duration, in months
   int dur();
 
-  /// Returns the number of months between the simulation start
+  /// Returns the number of time steps between the simulation start
   /// time and another time stamp.  
   ///
-  /// @param year of alternate time stamp
-  /// @param month of alternate time stamp
+  /// @param year of alternate time stamp (must be greater than 0)
+  /// @param month of alternate time stamp (must be greater than 0)
   ///
   /// @return the difference between the simulation start time and another time
-  /// @warning Only valid for times that occur after the simulation start time.
   int CalcTimeDiff(int year, int month);
 
  private: