apacheGH-44502: [R] Negative fractional dates must be converted to integers by floor, not trunc (apache#46873)

### Rationale for this change

Sub-day precision Dates specified as negative fractional values converted to wrong value if not floored first

### What changes are included in this PR?

Ensure they are converted with `floor()`

### Are these changes tested?

Yes

### Are there any user-facing changes?

Yes
* GitHub Issue: apache#44502

Authored-by: Nic Crane <[email protected]>
Signed-off-by: Nic Crane <[email protected]>

Author: thisisnic

r/src/r_to_arrow.cpp

@@ -595,11 +595,13 @@ class RPrimitiveConverter<T, enable_if_t<is_date_type<T>::value>>
     return VisitVector(it, size, append_null, append_value);
   }
 
-  static int FromRDate(const Date32Type*, double from) { return static_cast<int>(from); }
+  static int FromRDate(const Date32Type*, double from) {
+    return static_cast<int>(std::floor(from));
+  }
 
   static int64_t FromRDate(const Date64Type*, double from) {
     constexpr int64_t kMilliSecondsPerDay = 86400000;
-    return static_cast<int64_t>(from * kMilliSecondsPerDay);
+    return static_cast<int64_t>(std::floor(from * kMilliSecondsPerDay));
   }
 
   static int FromPosixct(const Date32Type*, double from) {

r/tests/testthat/test-Array.R

@@ -1397,3 +1397,9 @@ test_that("Can convert R integer/double to decimal (ARROW-11631)", {
     "Conversion to decimal from non-integer/double"
   )
 })
+
+test_that("Array handles negative fractional dates correctly (GH-46873)", {
+  d <- as.Date(-0.1)
+  arr <- arrow_array(d)
+  expect_equal(as.vector(arr), as.Date("1969-12-31"))
+})