Date picker widget not working w calculated expression

[FikriMilano]

Describe the bug
There is 2 issue to this:

1. The behavior_calculated_expression.json is not complete, missing unitOptions and the initial.valueQuantity.value, please use the JSON below to solve this first point
2. The Date picker widget in behavior_calculated_expression.json doesn't show the right value after it's calculated-expression was evaluated

After investigation, this code is the problem for point 2, it involves date formatting:
https://github.com/google/android-fhir/pull/1897/files#r1319542076

Component
SDC library and catalog app

To Reproduce
Steps to reproduce the behavior:

1. Catalog app
2. Click behavior tab
3. Click calculated expression
4. Fill in the Age years quantity widget
5. The date picker widget won't get updated

Expected behavior
The Date picker widget should be updated if the Age years quantity widget is filled

Demo

Screen_recording_20230908_160436.webm

Smartphone (please complete the following information):

• Device: [e.g. Pixel4a emulator]
• Android version: [e.g. Settings -> About phone -> Android version]
• Build number: [e.g. Settings -> About phone -> Build number]
• Version [e.g. 22]

Additional context

• I was attempting to fix this, but there's actually more to the issue (I'm referring to point 2):
• More context to the ticket enable when expression can access variable #2132 (comment)
• Use this complete behavior_calculated_expression.json

{
  "resourceType": "Questionnaire",
  "item": [
    {
      "linkId": "a-birthdate",
      "text": "Birth Date",
      "type": "date",
      "extension": [
        {
          "url": "http://hl7.org/fhir/uv/sdc/StructureDefinition/sdc-questionnaire-calculatedExpression",
          "valueExpression": {
            "language": "text/fhirpath",
            "expression": "%resource.repeat(item).where(linkId='a-age-years' and answer.empty().not()).select(today() - answer.value)"
          }
        }
      ]
    },
    {
      "linkId": "a-age-years",
      "text": "Age years",
      "type": "quantity",
      "extension": [
        {
          "url": "http://hl7.org/fhir/StructureDefinition/questionnaire-unitOption",
          "valueCoding": {
            "system": "http://unitsofmeasure.org",
            "code": "years",
            "display": "years"
          }
        }
      ],
      "initial": [
        {
          "valueQuantity": {
            "value": 0,
            "unit": "years",
            "system": "http://unitsofmeasure.org",
            "code": "years"
          }
        }
      ]
    }
  ]
}

Would you like to work on the issue?
I think @PallaviGanorkar is the right person for this

[jingtang10]

Can we also fix this questionnaire so that we're calculating age from date of birth? that seems like a much more reasonable use case.

[f-odhiambo]

@maimoonak Is working on this and will raise a PR soon