fix posixct conv back and forth

src/rust/src/conversion_r_to_s.rs

@@ -219,7 +219,7 @@ fn recursive_robjname2series_tree(x: &Robj, name: &str) -> pl::PolarsResult<Seri
             //todo this could probably in fewer allocations
             let dt = pl::DataType::Datetime(pl::TimeUnit::Milliseconds, tz);
             Ok(SeriesTree::Series(
-                (s.cast(&pl::DataType::Int64)? * 1_000i64).cast(&dt)?,
+                ((s * 1000f64).cast(&pl::DataType::Int64)?).cast(&dt)?,
             ))
         }
         Ok(SeriesTree::Series(s)) if x.inherits("Date") => {

src/rust/src/conversion_s_to_r.rs

@@ -186,19 +186,19 @@ pub fn pl_series_to_list(
                 }),
 
             Datetime(tu, opt_tz) => {
-                let tu_i64: i64 = match tu {
-                    pl::TimeUnit::Nanoseconds => 1_000_000_000,
-                    pl::TimeUnit::Microseconds => 1_000_000,
-                    pl::TimeUnit::Milliseconds => 1_000,
+                let tu_f64: f64 = match tu {
+                    pl::TimeUnit::Nanoseconds => 1_000_000_000.0,
+                    pl::TimeUnit::Microseconds => 1_000_000.0,
+                    pl::TimeUnit::Milliseconds => 1_000.0,
                 };
 
                 //resolve timezone
                 let tz = opt_tz.as_ref().map(|s| s.as_str()).unwrap_or("");
-                s.cast(&Int64)?
-                    .i64()
+                s.cast(&Float64)?
+                    .f64()
                     .map(|ca| {
                         ca.into_iter()
-                            .map(|opt| opt.map(|val| (val / tu_i64) as f64))
+                            .map(|opt| opt.map(|val| val / tu_f64))
                             .collect_robj()
                     })
                     // TODO set_class and set_attrib reallocates the vector, find some way to modify without.

tests/testthat/test-expr_datetime.R

@@ -18,6 +18,18 @@ test_that("pl$lit posix", {
     pl$lit(as.POSIXct("2022-01-01", tz = "GMT"))$to_r(),
     as.POSIXct("2022-01-01", tz = "GMT")
   )
+
+
+  x = as.POSIXct(
+    c(
+      "2020-01-01 13:45:48.343", "2020-01-01 13:45:48.815"
+      , "2020-01-01 13:45:49.289", "2020-01-01 13:45:49.974"
+      , "2020-01-01 13:45:51.190", "2020-01-01 13:45:51.631"
+    ), tz = "UTC"
+  )
+  expect_identical(pl$lit(x)$to_r(), x)  #preserve millisecond precision
+  expect_failure(expect_identical(pl$lit(x)$to_r(), x+0.001)) #control, detect 1ms offset
+
 })
 
 
@@ -846,3 +858,7 @@ test_that("$dt$time()", {
     c(0.00e+00, 2.16e+13, 4.32e+13, 6.48e+13, 0.00e+00)
   )
 })
+
+
+
+