date: Allow using a custom format string

This commit adds an optional string positional argument which is used a
custom format string for the date.

Base/usr/share/man/man1/date.md

@@ -5,7 +5,7 @@ date - print or set the system date and time
 ## Synopsis
 
 ```**sh
-$ date [--set date] [--unix] [--iso-8601] [--rfc-3339] [--rfc-5322]
+$ date [--set date] [--unix] [--iso-8601] [--rfc-3339] [--rfc-5322] [format-string]
 ```
 
 ## Description
@@ -21,12 +21,19 @@ or print the system date and time in various formats.
 * `-r`, `--rfc-3339`: Print date in RFC 3339 format
 * `-R`, `--rfc-5322`: Print date in RFC 5322 format
 
+## Arguments
+
+* `format-string`: Custom format to print the date in. Must start with a '+' character.
+
 ## Examples
 
 ```sh
 # Print the current date and time in ISO 8601 format
 $ date --iso-8601
 
+# Print the current date in a custom format
+$ date +%Y-%m-%d
+
 # Set date to 1610017485 (UNIX time)
 $ date -s 1610017485
 ```

Userland/Utilities/date.cpp

@@ -20,6 +20,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     bool print_rfc_3339 = false;
     bool print_rfc_5322 = false;
     const char* set_date = nullptr;
+    StringView format_string;
 
     Core::ArgsParser args_parser;
     args_parser.add_option(set_date, "Set system date and time", "set", 's', "date");
@@ -27,6 +28,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     args_parser.add_option(print_iso_8601, "Print date in ISO 8601 format", "iso-8601", 'i');
     args_parser.add_option(print_rfc_3339, "Print date in RFC 3339 format", "rfc-3339", 'r');
     args_parser.add_option(print_rfc_5322, "Print date in RFC 5322 format", "rfc-5322", 'R');
+    args_parser.add_positional_argument(format_string, "Custom format to print the date in", "format-string", Core::ArgsParser::Required::No);
     args_parser.parse(arguments);
 
     if (set_date != nullptr) {
@@ -43,15 +45,21 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
         return 0;
     }
 
-    // FIXME: this should be improved and will need to be cleaned up
-    // when additional output formats and formatting is supported
-    if (print_unix_date && print_iso_8601 && print_rfc_3339 && print_rfc_5322) {
-        warnln("date: multiple output formats specified");
+    if (print_unix_date + print_iso_8601 + print_rfc_3339 + print_rfc_5322 + !format_string.is_null() > 1) {
+        warnln("date: Multiple output formats specified");
         return 1;
     }
 
     auto date = Core::DateTime::now();
-    if (print_unix_date) {
+    if (!format_string.is_null()) {
+        // FIXME: If the string argument does not start with a '+' sign, POSIX says
+        //        we should parse that as a date, and set the system time to it.
+        if (format_string.length() == 0 || format_string[0] != '+') {
+            warnln("date: Format string must start with '+'");
+            return 1;
+        }
+        outln("{}", date.to_string(format_string.substring_view(1)));
+    } else if (print_unix_date) {
         outln("{}", date.timestamp());
     } else if (print_iso_8601) {
         outln("{}", date.to_string("%Y-%m-%dT%H:%M:%S%:z"));