In markdown files, Front-matter is configured with yaml syntax configuration. Rmarkdown is one type markdonw type used to generate reports with Rlanguage

This tutorial covers how to add current date in yaml formatter in rmarkdown files

Suppose you have yaml front matter as follows

---
date: 2021-07-16T00:00:00-00:00
title: "YAML -  Current Date in rmarkdown"
output: html_document
author: Renuka
--- 

You want to replace date field with current date produced with r expression

IN R language, Sys.Date() returns the current date in yyyy-mm-dd

---
date: `R Sys.Date()`
title: "YAML -  Current Date in rmarkdown"
output: html_document
author: Renuka
--- 

Generates the

---
date: 2021-07-16
title: "YAML -  Current Date in rmarkdown"
output: html_document
author: Renuka
--- 

if you want different format in reports, You can use format function with desired format.

format(Sys.Date(), "%d-%m-%Y") - 16-07-2021
format(Sys.Date(), "%d %B %Y") - 16 July 2021

The same can be using R format function in yaml syntax as follows

---
date: `r format(Sys.Date(), "%d %B %Y")`
title: "YAML -  Current Date in rmarkdown"
output: html_document
author: Renuka
--- 

Please note R expression should be enclosed in ` quote.