What is it called when you use a phrase to determine a date?

What's it called when you can use English language to explain a date?

For example, on today's date... the user enters the quoted phrase and it = the date:

• "this Saturday" = Saturday, September 19, 2020
• "next Saturday" = Saturday, September 26, 2020
• "next Monday at 3pm" = Monday, September 21, 2020 at 3:00PM

What is the name for this type of input? I use an app called Todoist and I would like to replicate it in a program I have.