diff --git a/src/commands/CmdCalendar.cpp b/src/commands/CmdCalendar.cpp
index b45712f95..b7f956288 100644
--- a/src/commands/CmdCalendar.cpp
+++ b/src/commands/CmdCalendar.cpp
@@ -127,12 +127,12 @@ int CmdCalendar::execute (std::string& output)
 
     // YYYY.
     else if (Lexer::isAllDigits (arg) && arg.length () == 4)
-      argYear = strtol (arg.c_str (), NULL, 10);
+      argYear = std::stoi (arg);
 
     // MM.
     else if (Lexer::isAllDigits (arg) && arg.length () <= 2)
     {
-      argMonth = strtol (arg.c_str (), NULL, 10);
+      argMonth = std::stoi (arg);
       if (argMonth < 1 || argMonth > 12)
         throw format (STRING_CMD_CAL_BAD_MONTH, arg);
     }
@@ -569,7 +569,7 @@ std::string CmdCalendar::renderMonths (
                 task.has ("due"))
             {
               std::string due = task.get ("due");
-              ISO8601d duedmy (strtol (due.c_str(), NULL, 10));
+              ISO8601d duedmy (static_cast <time_t> (std::stoul (due)));
 
               if (duedmy.day   () == d           &&
                   duedmy.month () == months[mpl] &&