Add date and today

Add two new functions for working with dates, without having to care
about times.

book/src/date-and-time.md

@@ -13,7 +13,7 @@ now() + 40 days
 now() - 1 million seconds
 
 # How many days are left until September 1st?
-datetime("2024-11-01 12:30:00") - now() -> days
+date("2024-11-01") - today() -> days
 
 # What time is it in Nepal right now?
 now() -> tz("Asia/Kathmandu")  # use tab completion to find time zone names
@@ -58,7 +58,9 @@ April 1st"?
 The following functions are available for date and time handling:
 
 - `now() -> DateTime`: Returns the current date and time.
-- `datetime(input: String) -> DateTime`: Parses a string into a `DateTime` object.
+- `today() -> DateTime`: Returns the current date at midnight (in the local time).
+- `datetime(input: String) -> DateTime`: Parses a string (date and time) into a `DateTime` object.
+- `date(input: String) -> DateTime`: Parses a string (only date) into a `DateTime` object.
 - `format_datetime(format: String, dt: DateTime) -> String`: Formats a `DateTime` object as a string. See [this page](https://docs.rs/chrono/latest/chrono/format/strftime/index.html#specifiers) for possible format specifiers.
 - `tz(tz: String) -> Fn[(DateTime) -> DateTime]`: Returns a timezone conversion function, typically used with the conversion operator (`datetime -> tz("Europe/Berlin")`)
 - `local(dt: DateTime) -> DateTime`: Timezone conversion function targeting the users local timezone (`datetime -> local`)
@@ -85,3 +87,11 @@ zone is specified, the local time zone is used.
 | `%Y/%m/%d %I:%M:%S%.f %p` | same, but with `/` separator |
 | `%Y-%m-%d %I:%M %p` | `2024-02-10 12:30 PM`<br>`2024-02-10 06:30 AM -0600`<br>`2024-02-10 07:30 AM US/Eastern` |
 | `%Y/%m/%d %I:%M %p` | same, but with `/` separator |
+
+The `date` function supports the following formats. It returns a `DateTime` object with the time set to midnight in the
+specified timezone (or the local timezone if no timezone is specified).
+
+| Format | Examples |
+| ------ | ------- |
+| `%Y-%m-%d` | `2024-02-10`<br>`2024-02-10 +0100`<br>`2024-02-10 Europe/Berlin` |
+| `%Y/%m/%d` | `2024/02/10`<br>`2024/02/10 +0100`<br>`2024/02/10 Europe/Berlin` |

numbat/modules/datetime/functions.nbt

@@ -1,11 +1,23 @@
+use core::strings
 use units::si
 
 fn now() -> DateTime
+
 fn datetime(input: String) -> DateTime
 fn format_datetime(format: String, input: DateTime) -> String
+
 fn get_local_timezone() -> String
 fn tz(tz: String) -> Fn[(DateTime) -> DateTime]
 let local: Fn[(DateTime) -> DateTime] = tz(get_local_timezone())
 let UTC: Fn[(DateTime) -> DateTime] = tz("UTC")
+
 fn unixtime(input: DateTime) -> Scalar
 fn from_unixtime(input: Scalar) -> DateTime
+
+fn _today_str() = format_datetime("%Y-%m-%d", now())
+fn today() -> DateTime = datetime("{_today_str()} 00:00:00")
+
+fn date(input: String) -> DateTime =
+  if str_contains(input, " ")
+    then datetime(str_replace(input, " ", " 00:00:00 "))
+    else datetime("{input} 00:00:00")