Add date_modify filter

README.md

@@ -133,6 +133,9 @@ Filters allow you to modify variables in a template. Filters are applied to vari
 - `date` converts a date to the specified format, [see reference](https://day.js.org/docs/en/display/format).
 	- `date:"YYYY-MM-DD"` converts a date to "YYYY-MM-DD".
 	- Use `date:("outputFormat", "inputFormat")` to specify the input format, e.g. `"12/01/2024"|date:("YYYY-MM-DD", "MM/DD/YYYY")` parses "12/01/2024" and returns `"2024-12-01"`.
+- `date_modify` modifies a date by adding or subtracting a specified amount of time, [see reference](https://day.js.org/docs/en/manipulate/add).
+	- `"2024-12-01"|date_modify:"+1 year"` returns `"2025-12-01"`
+	- `"2024-12-01"|date_modify:"- 2 months"` returns `"2024-10-01"`
 - `first` returns the first element of an array as a string.
 	- `["a","b","c"]|first` returns `"a"`.
 	- If the input is not an array, it returns the input unchanged.

src/utils/filters.ts

@@ -30,13 +30,15 @@ import { wikilink } from './filters/wikilink';
 import { template } from './filters/template';
 import { map } from './filters/map';
 import { strip_tags } from './filters/strip_tags';
+import { date_modify } from './filters/date_modify';
 
 export const filters: { [key: string]: FilterFunction } = {
 	blockquote,
 	camel,
 	capitalize,
 	callout,
 	date,
+	date_modify,
 	first,
 	footnote,
 	image,
@@ -55,7 +57,7 @@ export const filters: { [key: string]: FilterFunction } = {
 	split,
 	strip_attr,
 	strip_md,
-	stripmd: strip_md, // stripmd is an alias for strip_md
+	stripmd: strip_md, // an alias for strip_md
 	strip_tags,
 	table,
 	template,

src/utils/filters/date_modify.ts

@@ -0,0 +1,37 @@
+import dayjs from 'dayjs';
+
+export const date_modify = (str: string, param?: string): string => {
+	if (!param) {
+		console.error('date_modify filter requires a parameter');
+		return str;
+	}
+
+	let date = dayjs(str);
+	if (!date.isValid()) {
+		console.error('Invalid date for date_modify filter:', str);
+		return str;
+	}
+
+	// Remove any surrounding quotes and trim whitespace
+	param = param.replace(/^["']|["']$/g, '').trim();
+
+	// Updated regex to allow for optional spaces and plural units
+	const regex = /^([+-])\s*(\d+)\s*(\w+)s?$/;
+	const match = param.match(regex);
+
+	if (!match) {
+		console.error('Invalid format for date_modify filter:', param);
+		return str;
+	}
+
+	const [, operation, amount, unit] = match;
+	const numericAmount = parseInt(amount, 10);
+
+	if (operation === '+') {
+		date = date.add(numericAmount, unit as any);
+	} else {
+		date = date.subtract(numericAmount, unit as any);
+	}
+
+	return date.format('YYYY-MM-DD');
+};