Implement calendar duration conversions for toQuantity(unit) and convertsToQuantity(unit)

[piotrszul]

Overview

The toQuantity(unit) and convertsToQuantity(unit) functions currently support calendar duration units (years, months, weeks, days, hours, minutes, seconds, milliseconds) but do not perform conversions between them. This issue tracks the implementation of calendar duration conversion capabilities.

Current Implementation

The current implementation:

• Accepts calendar duration literals (e.g., '4 days', '1 year', '3 months')
• Performs exact string matching on unit codes (e.g., '4 days'.toQuantity('days') works)
• Returns empty when units don't match (e.g., '4 days'.toQuantity('weeks') returns empty)
• Does not convert between calendar duration units

Requested Enhancement

Implement calendar duration conversions so that:

• Definite duration units can be converted (e.g., days ↔ hours, minutes ↔ seconds)
• Proper handling of indefinite durations (years, months) per FHIRPath specification
• Conversion follows the calendar duration conversion factors specified in FHIRPath spec

Calendar Duration Conversion Factors (from FHIRPath spec)

From the FHIRPath specification (section on Quantity Conversion):

Duration	Conversion Factor
1 year	365 days
1 month	30 days
1 week	7 days
1 day	24 hours
1 hour	60 minutes
1 minute	60 seconds
1 second	1000 milliseconds

Note: Years and months are indefinite durations and their conversions may need special handling.

Examples

'1 week'.toQuantity('days')       // Should return 7 days
'24 hours'.toQuantity('days')     // Should return 1 day
'60 minutes'.toQuantity('hours')  // Should return 1 hour

Implementation Notes

• Definite durations (day, hour, minute, second, millisecond) should support full conversion
• Indefinite durations (year, month) require careful consideration per FHIRPath spec
• Consider interaction with UCUM unit conversion (issue Implement full UCUM unit conversion for toQuantity(unit) and convertsToQuantity(unit) #2504)
• Update documentation about which calendar duration conversions are supported
• Update test cases once implemented

Related

• Parent issue: Support all type conversion functions #2391
• Related issue: Implement full UCUM unit conversion for toQuantity(unit) and convertsToQuantity(unit) #2504 (UCUM unit conversion)
• Comparison exclusions in config.yaml related to calendar durations (lines 203-218)
• Related files:
  • FhirpathQuantity.java (calendar duration handling)
  • CalendarDurationUnit.java (calendar unit definitions)
  • ConversionLogic.java:152-193
  • ValidationLogic.java:123-183