Add timedelta() template filter and timestamp global

Introduce a "timestamp" template global variable with the date/time at
which the "!render" command was issued, and a "timedelta()" filter that
may be used to get new timestamps calculated by adding a time duration
(in seconds, minutes, hours, etc.) to an existing one.

This allows calculating texts like "from September 20 up to October 7"
in templates.

Cargo.toml

@@ -21,6 +21,7 @@ async-process = "2.2"
 minijinja = { version = "2.0.1", features = ["builtins", "debug", "deserialization", "json", "loader", "std_collections"] }
 minijinja-contrib = { version = "2.0.1", features = ["datetime", "rand"] }
 minijinja-autoreload = "2.2.0"
+time = "0.3.36"
 
 [features]
 default = [

doc/example_config/template.md

@@ -1,7 +1,7 @@
 ---
-title: "#{{ now() | dateformat(format='[week_number]') }}: This Week in X"
+title: "#{{ timestamp | dateformat(format='[week_number]') }}: This Week in X"
 author: {{ editor }}
-date: {{ now() | datetimeformat(format="iso") }}
+date: {{ timestamp | datetimeformat(format="iso") }}
 tags: {{ projects }}
 categories: ["weekly-update"]
 draft: false
@@ -22,6 +22,22 @@ draft: false
  #   used for processing the template. This is useful for writing custom
  #   templates.
  #
+ # - On top of the MiniJinja built-ins (including the minijinja-contrib ones),
+ #   the following globals are available during template processing:
+ #
+ #     editor: display name of the editor that issued the !render command.
+ #     timestamp: date/time of the !render command (see more below).
+ #     sections: sections containing projects and news items.
+ #     projects: names of the projects that have news items.
+ #     config: contents of the TOML configuration file.
+ #
+ # - The "timestamp" global contains the date and time at which the !render
+ #   command was issued. The timedelta() filter can be used to derive new
+ #   date/time values by adding and subtracting time periods, e.g.:
+ #   "now() | timedelta(weeks=-1)" obtains the date/time for one week
+ #   ago. Accepted parameters are "seconds", "minutes", "hours", "days",
+ #   "weeks", "months", and "years".
+ #
  # - Macros can be used to avoid repeating template fragments. See below
  #   for an example macro to handle both section and project news.
  #
@@ -50,7 +66,10 @@ draft: false
   {%- endfor %} {#- news_items #}
 {%- endmacro %}
 
-Update on what happened across the X project in the week from {{timespan}}.
+Update on what happened across the X project in the week from {{
+  timestamp | timedelta(weeks = -1) | dateformat(format="[month repr:long] [day padding:none]")
+}} to {{
+  timestamp | dateformat(format="[month repr:long] [day padding:none]") }}.
 
 {%- for key, entry in sections | dictsort %}
 

src/render.rs

@@ -33,6 +33,39 @@ pub struct RenderResult {
     pub videos: Vec<(String, OwnedMxcUri)>,
 }
 
+fn template_filter_timedelta(
+    timestamp: minijinja::value::ViaDeserialize<time::OffsetDateTime>,
+    options: minijinja::value::Kwargs,
+) -> Result<minijinja::Value, minijinja::Error> {
+    let mut duration = time::Duration::seconds(0);
+    if let Some(seconds) = options.get::<Option<f64>>("seconds")? {
+        duration = duration.saturating_add(time::Duration::seconds_f64(seconds));
+    }
+    if let Some(minutes) = options.get::<Option<i64>>("minutes")? {
+        duration = duration.saturating_add(time::Duration::minutes(minutes));
+    }
+    if let Some(hours) = options.get::<Option<i64>>("hours")? {
+        duration = duration.saturating_add(time::Duration::hours(hours));
+    }
+    if let Some(days) = options.get::<Option<i64>>("days")? {
+        duration = duration.saturating_add(time::Duration::days(days));
+    }
+    if let Some(weeks) = options.get::<Option<i64>>("weeks")? {
+        duration = duration.saturating_add(time::Duration::days(weeks * 7));
+    }
+    if let Some(months) = options.get::<Option<i64>>("months")? {
+        duration = duration.saturating_add(time::Duration::days(months * 30));
+    }
+    if let Some(years) = options.get::<Option<i64>>("years")? {
+        duration = duration.saturating_add(time::Duration::days(years * 365));
+    }
+    options.assert_all_used()?;
+
+    Ok(minijinja::Value::from_serialize(
+        timestamp.saturating_add(duration),
+    ))
+}
+
 static RELOADER: LazyLock<minijinja_autoreload::AutoReloader> = LazyLock::new(|| {
     minijinja_autoreload::AutoReloader::new(|notifier| {
         use std::io::Read;
@@ -58,6 +91,7 @@ static RELOADER: LazyLock<minijinja_autoreload::AutoReloader> = LazyLock::new(||
 
         let mut env = minijinja::Environment::new();
         minijinja_contrib::add_to_environment(&mut env);
+        env.add_filter("timedelta", template_filter_timedelta);
         env.add_template_owned("template", text)?;
 
         notifier.watch_path(path, false);
@@ -247,6 +281,7 @@ pub fn render(
 
     let env = &*RELOADER.acquire_env()?;
     let rendered = env.get_template("template")?.render(minijinja::context! {
+        timestamp => time::OffsetDateTime::now_utc(),
         sections => render_sections,
         projects => project_names,
         config => config,