How to create an expiration date that's 48 hours from the global variable current time?

Hi! I'm trying to allow the user to create an object that has an expiration date 48 hours from the current time and have that time variable be stored in Firebase. How do I do this? For example, they create a coupon variable and then when the document is created, the expiration date of the coupon is set to be 48 hours from the current time and stored in Firebase. When I try to set the time variable with the Global Variables, the only option is the global variable of the current local time.