DatePicker spec (#1537)

## 📝 Changes

- DatePicker Spec

Co-authored-by: Kevin Liu <[email protected]>

documentation/specs/DatePicker.md

@@ -0,0 +1,111 @@
+# `DatePicker` Component Specification
+
+## Overview
+
+DatePicker combine a date field and a calendar popver to allow users to enter or select a date.
+
+### Use Cases
+
+- Use a DatePciker when you want to provide a view that allows the users to select a date.
+
+### Features
+
+- Supports setting dates availability
+- Supports being controlled
+
+### Prior Art
+
+- [Aria's `useDatePicker`](https://react-spectrum.adobe.com/react-aria/useDatePicker.html)
+- [Aria's `useDateRangePicker`](https://react-spectrum.adobe.com/react-aria/useDateRangePicker.html)
+- [Paste `Date Picker`](https://paste.twilio.design/components/date-picker)
+
+---
+
+## Design
+
+`DatePicker` will use `useDatePicker` and `useDateRangePicker` from `React Aria` to helps achieve accessible date picker.
+
+A `DatePicker` composes several other components to product a composite element that can be used to enter dates with keyboard, or select them on a calendar. The component consist of `DatePicker` wrapper, `DatePicker.Trigger` to open a `DatePicker.Overlay` containing a `Calendar` and `DateField` for selecting and inputing dates.
+
+### API
+
+```ts
+type DatePickerProps = {
+  /**
+   * The content to display as the label.
+   */
+  label: ReactNode;
+  /**
+   * The minimum allowed date that a user may select.
+   */
+  minValue: DateValue;
+  /**
+   * The maximum allowed date that a user may select.
+   */
+  maxValue: DateValue;
+  /**
+   * Callback that is called for each date of the calendar. If it returns
+   * true, then the date is unavailable.
+   */
+  isDateUnavailable: (date: DateValue) => boolean;
+  /**
+   * A placeholder date that influences the format of the placeholder shown
+   * when no value is selected. Defaults to today's date at midnight.
+   */
+  placeholderValue: DateValue;
+  /**
+   * Whether the input is disabled.
+   */
+  isDisabled: boolean;
+  /**
+   * Whether the input can be selected but not changed by the user.
+   */
+  isReadOnly: boolean;
+  /**
+   * Whether user input is required on the input before form submission.
+   */
+  isRequired: boolean;
+  /**
+   * Whether the input value is invalid.
+   */
+  isInvalid: boolean;
+  /**
+   * An error message for the field.
+   */
+  errorMessage: ReactNode;
+};
+```
+
+### Example Usage
+
+_Standalone_:
+
+```tsx
+import { DatePicker } from "@easypost/easy-ui/DatePicker";
+import { Calendar } from "@easypost/easy-ui/Calendar";
+import { DateField } from "@easypost/easy-ui/DateField";
+
+function PageWithDatePicker() {
+  return (
+    <DatePicker>
+      <DatePicker.Trigger>
+        <Button>01/01/2024 - 02/01/2024</Button>
+      </DatePicker.Trigger>
+      <DatePicker.Overlay>
+        <DateField />
+        <Calendar />
+      </DatePicker.Overlay>
+    </DatePicker>
+  );
+}
+```
+
+## Behavior
+
+### Accessibility
+
+- Accessibility are handle by `useDatePicker` and `useDateRangePicker` from `Reat Aria`.
+
+### Dependencies
+
+- `useDatePicker` and `useDateRangePicker` from `Reat Aria`