Modify a value in the README during the CI

Hello, I’m trying to change the date during the CI in the FIRST UPDATE part of my README.md:

# CONKY

## FIRST UPDATE

Date: 2018-01-01


## INTRODUCTION

Docker image of :

- [conky](https://github.com/brndnmtthws/conky)

## PREREQUISITES

Use [docker](https://www.docker.com)

here is the date part which I use in my gitlab-ci.yml but I would like to use the variable CI_JOB_TIMESTAMP

date:
  stage: date
  allow_failure: false
  script:
    - echo -e '\033[36;1m ******* DATE ******** \033[0m'
    - "date=$(date +\"%Y-%m-%d\") | sed -i \"s/^Date: .*/Date: $date/\" README.md"
  only:
    - dev

it works locally but not in my CI because the value remains the same.

thank you in advance for your help.