Set a date field value to a specific weekday based on another date field

[Hotaru]

Hi guys!

Hope someone can help with this question:

We have 2 date fields right now, and need to automatically set date B according to what date A is. Both date A and B would be custom fields.

Date B should be Monday or Thursday specifically depending on which one is closest to date A, for example:

• if date A is 7/17/2019, date B should be set to 7/18/2019
• if date A is 7/19/2019, date B should be set to 7/22/2019

We have Script Runner, but i'm honestly not sure how to script this to achieve this.

Many thanks in advance!