Add license for date.h

from https://github.com/HowardHinnant/date/blob/master/include/date/date.h

resources/licenses/howard-hinnant-date.txt

@@ -0,0 +1,30 @@
+The MIT License (MIT)
+
+Copyright (c) 2015, 2016, 2017 Howard Hinnant
+Copyright (c) 2016 Adrian Colomitchi
+Copyright (c) 2017 Florian Dang
+Copyright (c) 2017 Paul Thompson
+Copyright (c) 2018, 2019 Tomasz Kamiński
+Copyright (c) 2019 Jiangang Zhuang
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+
+Our apologies.  When the previous paragraph was written, lowercase had not yet
+been invented (that would involve another several millennia of evolution).
+We did not mean to shout.

src/widgets/settingspages/AboutPage.cpp

@@ -126,6 +126,9 @@ AboutPage::AboutPage()
             addLicense(form.getElement(), "Fluent icons",
                        "https://github.com/microsoft/fluentui-system-icons",
                        ":/licenses/fluenticons.txt");
+            addLicense(form.getElement(), "Howard Hinnant's date.h",
+                       "https://github.com/microsoft/fluentui-system-icons",
+                       ":/licenses/fluenticons.txt");
         }
 
         // Attributions