do not allow scheduling events in the past

app/src/main/java/com/simplemobiletools/calendar/activities/EventActivity.java

@@ -188,6 +188,11 @@ public class EventActivity extends AppCompatActivity implements DBHelper.DBOpera
             return;
         }
 
+        if (DateTime.now().isAfter(mEventStartDateTime.getMillis())) {
+            Utils.showToast(getApplicationContext(), R.string.start_in_past);
+            return;
+        }
+
         final DBHelper dbHelper = DBHelper.newInstance(getApplicationContext(), this);
         final String description = mDescriptionET.getText().toString().trim();
         final int reminderMinutes = getReminderMinutes();

app/src/main/res/values/strings.xml

@@ -10,6 +10,7 @@
     <string name="delete">Delete</string>
     <string name="title_empty">Title cannot be empty</string>
     <string name="end_before_start">The event cannot end earlier than it starts</string>
+    <string name="start_in_past">You cannot schedule events in the past</string>
     <string name="event_added">Event added successfully</string>
     <string name="event_updated">Event updated successfully</string>
     <string name="reminder">Reminder</string>