how to record file opened status from multiple user files to master file ?

Hi all,

Need a Quick help here to achieve the following,

There are multiple users who access one of the template file that i have given to them. It is individual file, not shared file.
Once each user opens the file, time stamp will be published in A1 in their own individual file. Attached ( individual file).


The need here is, once each user opens the file the master file should get updated against respective user id as "Y"
Challenge here is , every user would be opening the file at same time. So this should not interupt our requirement.
Attached (Master File )

Attached both Individual file and Master file.

Please help.