Add EF.Functions translations for the remaining ClickHouse date/time functions

[alex-clickhouse]

Problem

PR #57 adds the ClickHouse toStartOf* family as EF.Functions extension methods. It closes #56.

Many other ClickHouse date/time functions stay unavailable from LINQ. These functions have no .NET equivalent, so an EF.Functions entry point is the correct shape for them.

This issue is different from #55. Issue #55 covers the translation of standard .NET members and methods, such as .Year and .AddDays(1). Those must translate without an EF.Functions call. This issue covers only the ClickHouse-specific functions.

Suggested work

Use the same shape as PR #57 for each function:

• an extension method on DbFunctions in ClickHouseDateTimeDbFunctionsExtensions
• a MethodInfo entry in ClickHouseDateTimeMethodTranslator
• a test in DateTimeFunctionsTranslationTests

Add the functions in this order. The order shows the expected value to users.

#	Functions	Notes
1	dateDiff, age	Add both. They give different results. Re-use the ClickHouseInterval enum from PR #57 for the unit. The return type is Int64, thus declare long.
2	dateTrunc	The general form of toStartOf*. Uses the same interval enum.
3	toLastDayOfMonth, toLastDayOfWeek	Companions to toStartOf*. The same translator can hold them.
4	toTimeZone, toUTCTimestamp, fromUTCTimestamp	Do this work after #53 is merged. The DateTimeOffset mapping changes the correct return type.
5	toWeek, toYearWeek, toISOWeek, toISOYear	Week numbers with a mode argument. .NET has no equivalent.
6	toUnixTimestamp, fromUnixTimestamp	toUnixTimestamp returns UInt32.
7	formatDateTime	The only way to format on the server. ToString(format) cannot translate.
8	changeYear, changeMonth, changeDay, changeHour, changeMinute, changeSecond	Six methods with one shape.
9	toRelativeYearNum ... toRelativeSecondNum	Integer bucket keys for GROUP BY.
10	dateName, monthName	Name extraction.

Out of scope

Do not add these functions now. They have a small audience. Add them if a user asks.

formatDateTimeInJodaSyntax, fromUnixTimestampInJodaSyntax, toModifiedJulianDay, fromModifiedJulianDay, toDaysSinceYearZero, fromDaysSinceYearZero, nowInBlock, serverTimezone, UTCTimestamp, localtime, addInterval, addTupleOfIntervals, timeSlot, timeSlots, toYYYYMMDD, toYYYYMMDDhhmmss, YYYYMMDDToDate.

Measured ClickHouse behaviour

These results come from a ClickHouse 26.7.1 server. Use them when you write the tests.

Return types are narrow. The translator must attach the correct type mapping, or the read fails:

Expression	Return type
toYear(...), toDayOfYear(...), toMillisecond(...)	UInt16
toMonth(...), toDayOfMonth(...), toHour(...), toDayOfWeek(...)	UInt8
dateDiff(...)	Int64
toUnixTimestamp(...)	UInt32
toStartOfMonth(DateTime)	Date, not DateTime
toDate(...) - toDate(...)	Int32, a count of days

dateDiff and age give different results. dateDiff counts the unit boundaries between the two values. age gives the number of full units.

SELECT dateDiff('month', toDate('2021-12-29'), toDate('2022-01-01'));  -- 1
SELECT age('month', toDate('2021-12-29'), toDate('2022-01-01'));       -- 0

Other measured values:

SELECT toLastDayOfMonth(toDate('2026-02-03'));   -- 2026-02-28
SELECT toLastDayOfWeek(toDate('2026-08-12'));    -- 2026-08-15, a Saturday
SELECT toISOWeek(toDate('2026-08-12'));          -- 33
SELECT toWeek(toDate('2026-08-12'));             -- 32
SELECT toTimeZone(toDateTime('2026-08-12 12:00:00', 'UTC'), 'Europe/Paris');  -- 2026-08-12 14:00:00

toLastDayOfWeek uses mode 0 by default, and mode 0 starts the week on Sunday. This agrees with toStartOfWeek, which PR #57 documents. Be careful: toDayOfWeek also has a mode 0, but that mode starts the week on Monday. The default mode is not the same across the family.

Related change: the evaluatable expression filter

ClickHouseEvaluatableExpressionFilter.IsEvaluatableExpression has one hard-coded type test for each class of EF.Functions methods. PR #57 adds the second test:

MethodCallExpression methodCallExpression when methodCallExpression.Method.DeclaringType ==
                                               typeof(ClickHouseJsonDbFunctionsExtensions) => false,
MethodCallExpression methodCallExpression when methodCallExpression.Method.DeclaringType ==
                                               typeof(ClickHouseDateTimeDbFunctionsExtensions) => false,

Each new class of EF.Functions methods adds one more test. If a contributor forgets the test, the calls evaluate on the client and become constants. The failure is silent.

Replace the tests with one general rule. There are two options:

• a marker attribute on the extensions class
• a static set of types, which each extensions class registers itself in

Make this change before a third class exists.