diff --git a/src/datetime.cc b/src/datetime.cc
index e82cf5caef97af88a533485fe8f72d0d97dfed61..e260cc71976b21df6a4ebaf365c56281c2edac28 100644
--- a/src/datetime.cc
+++ b/src/datetime.cc
@@ -37,18 +37,18 @@ time_units_cstr(TimeUnits timeUnit)
 }
 
 void
-set_timestat_date(const std::string &optarg)
+set_timestat_date(const std::string &p_optarg)
 {
   TimeStat timestatdate = TimeStat::UNDEF;
 
   // clang-format off
-  if      (optarg == "first")   timestatdate = TimeStat::FIRST;
-  else if (optarg == "last")    timestatdate = TimeStat::LAST;
-  else if (optarg == "middle")  timestatdate = TimeStat::MEAN;
-  else if (optarg == "midhigh") timestatdate = TimeStat::MIDHIGH;
+  if      (p_optarg == "first")   timestatdate = TimeStat::FIRST;
+  else if (p_optarg == "last")    timestatdate = TimeStat::LAST;
+  else if (p_optarg == "middle")  timestatdate = TimeStat::MEAN;
+  else if (p_optarg == "midhigh") timestatdate = TimeStat::MIDHIGH;
   // clang-format on
 
-  if (timestatdate == TimeStat::UNDEF) cdo_abort("option --%s: unsupported argument: %s", "timestat_date", optarg);
+  if (timestatdate == TimeStat::UNDEF) cdo_abort("option --%s: unsupported argument: %s", "timestat_date", p_optarg);
 
   CDO_Timestat_Date = timestatdate;
 }
diff --git a/src/datetime.h b/src/datetime.h
index ec510c7de685c88b182fe909582224e23221292b..61cda014e2849c710e4c6c54a3b7e86417fa2387 100644
--- a/src/datetime.h
+++ b/src/datetime.h
@@ -111,7 +111,7 @@ private:
 };
 
 CdiDateTime datetime_avg(int calendar, int ndates, const std::vector<CdiDateTime> &cdiDateTimes);
-void set_timestat_date(const std::string &optarg);
+void set_timestat_date(const std::string &p_optarg);
 
 void adjust_month_and_year(int &month, int &year);