javascript - How to ask the user to input a date when first open the website then save the date as a global value throughout the entire websites

I have a project that I need to build a hotel booking system website using HTML, PHP, JS, and NoSQL(firebase from Google), however, I need the user to input date as a current date then the inserted date will be used as a global value for the entire project.
For example, when the user opens the index.html, then the user will be asked to insert a date (let's say the date is 2019-12-25) as the current date. Then, the inserted date will be treated as "today", so when the user books a room in a hotel, then the system will save that the user booked the room in 2019-12-25.
How do you save the date as a global value for the entire project?
Thank you so much for your time, please let me know if my question is too broad or not clear.
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