docs: DatePicker fallback parser

articles/components/date-picker/date-formats.adoc

@@ -105,6 +105,20 @@ Other characters, such as separators (e.g., `/`, `.`, `-`) or spaces may be used
 Custom date patterns take precedence over the configured locale. When using both at the same time, the custom date pattern is used to display and parse dates.
 endif::[]
 
+== Server-Side Parsing in Flow
+
+Additionally, Date Picker allows you to provide a server-side parser that is used as a fallback when user input cannot be parsed using methods described in the previous sections. The parser is a function that receives the user-entered string and returns either a `Result<LocalDate>` object or an error message. The parsed date is then displayed like any other dates, using the date format pattern that was configured in the previous sections.
+
+The server-side parser can be used, for example, to support shortcuts for quickly entering dates, such as `+1` or `tomorrow`. In the following example, try entering `+1`, `-2`, and `foobar` to see the server-side parser in action:
+
+[.example]
+--
+[source,java]
+----
+include::{root}/src/main/java/com/vaadin/demo/component/datepicker/DatePickerFallbackParser.java[render,tags=snippet,indent=0]
+----
+--
+
 
 == Using Custom Functions in React & Lit
 

src/main/java/com/vaadin/demo/component/datepicker/DatePickerFallbackParser.java

@@ -0,0 +1,36 @@
+package com.vaadin.demo.component.datepicker;
+
+import com.vaadin.demo.DemoExporter; // hidden-source-line
+import com.vaadin.flow.component.datepicker.DatePicker;
+import com.vaadin.flow.component.orderedlayout.VerticalLayout;
+import com.vaadin.flow.data.binder.Result;
+import com.vaadin.flow.router.Route;
+
+import java.time.LocalDate;
+
+@Route("date-picker-fallback-parser")
+public class DatePickerFallbackParser extends VerticalLayout {
+    public DatePickerFallbackParser() {
+        DatePicker datePicker = new DatePicker("Select a date:");
+        // tag::snippet[]
+        datePicker.setFallbackParser((s) -> {
+            LocalDate now = LocalDate.now();
+            if (s.matches("\\+\\d+")) {
+                return Result
+                        .ok(now.plusDays(Integer.parseInt(s.substring(1))));
+            } else if (s.matches("-\\d+")) {
+                return Result
+                        .ok(now.minusDays(Integer.parseInt(s.substring(1))));
+            } else {
+                return Result.error("Invalid date format");
+            }
+        });
+        // end::snippet[]
+        datePicker.setHelperText("Format: +1, -2 for relative days");
+        add(datePicker);
+    }
+
+    public static class Exporter
+            extends DemoExporter<DatePickerFallbackParser> { // hidden-source-line
+    } // hidden-source-line
+}