fix dfr datetime conversion (#7264)

# Description

Closes #7257

This fixes an issue where dataframes would always try to convert
datetimes with milliseconds. Since the timeunit is passed in, I make use
of it and try to choose the appropriate divisor.


# User-Facing Changes


# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-command/src/dataframe/values/nu_dataframe/conversion.rs

@@ -498,7 +498,7 @@ pub fn create_column(
 
             Ok(Column::new(casted.name().into(), values))
         }
-        DataType::Datetime(_, _) => {
+        DataType::Datetime(time_unit, _) => {
             let casted = series.datetime().map_err(|e| {
                 ShellError::GenericError(
                     "Error casting column to datetime".into(),
@@ -508,15 +508,19 @@ pub fn create_column(
                     Vec::new(),
                 )
             })?;
-
             let values = casted
                 .into_iter()
                 .skip(from_row)
                 .take(size)
                 .map(|v| match v {
                     Some(a) => {
-                        // elapsed time in milliseconds since 1970-01-01
-                        let seconds = a / 1000;
+                        let unit_divisor = match time_unit {
+                            TimeUnit::Nanoseconds => 1_000_000_000,
+                            TimeUnit::Microseconds => 1_000_000,
+                            TimeUnit::Milliseconds => 1_000,
+                        };
+                        // elapsed time in nano/micro/milliseconds since 1970-01-01
+                        let seconds = a / unit_divisor;
                         let naive_datetime = match NaiveDateTime::from_timestamp_opt(seconds, 0) {
                             Some(val) => val,
                             None => {