Calendar functions

date_add(d, years: 1)
date_sub(d, days: n + 1)
date_trunc(d, month)
date_diff(start_date, end_date, day)

• Shifts require exactly one named unit: years, quarters, months, weeks, or days. Amounts are integer expressions, including negative and nullable values. Multiple units require nested calls; ordering matters.
• Truncation and differences accept year, quarter, month, week, day as a bare symbol or a quoted string. Differences also accept weeks. Unit symbols are contextual; ordinary columns named day or month are not reserved.
• Shifts and truncation preserve the first argument's date or timestamp type. Null values propagate, including a null shift amount.
• Month/quarter/year shifts clamp to the last valid day of the destination month: 2024-02-29 + 1 year = 2025-02-28. They do not preserve an end-of-month flag: 2024-02-29 + 1 month = 2024-03-29.
• date_diff(start, end, unit) returns an int64 count of calendar boundaries crossed, not elapsed complete durations. Both temporal arguments must have the same type. Reversing them reverses the sign. Weeks start Monday.
• Timestamp calculations use UTC civil time. BigQuery conversions explicitly use UTC; other targets use Strata's timezone-free timestamp SQL types. Sub-day units and timezone-aware arithmetic are outside this surface.

The typed plan retains each date call's base type for SQL generation. DuckDB/PostgreSQL interval results are cast back to DATE when needed. BigQuery uses DATE/DATETIME operations; Snowflake uses DATEADD/DATEDIFF. Snowflake week operations avoid dependence on the session's WEEK_START.

Diagnostics: E062 arity, E063 argument kind, E071 unsupported unit, E072 invalid keyword/unit shape, E073 mismatched date_diff temporal types. Untyped date calls cannot use the generic SQL function emitter.

Validation: tests/test_date_functions.py executes calendar and result-type checks on DuckDB and checks SQL emission for all four dialects. It does not execute against live BigQuery, Snowflake, or PostgreSQL services.