How can I create a datetime field and properly associate it to a Checkbox field?

Hi, I am new to all this, but I learn fast, so I just recently started to play with this and I found myself trying to track a simple process with a simple Checkbox, whenever that process for a Lead happens the users will click that Checkbox and save changes, so this process is kind of really important, and I want to track the leads that get to that part of the funnel periodically, for example, instead of see how many leads were created on July, I want to know how many Leads get to that part of the funnel on July, the matter is that so far I have been tracking the Leads for creation date but with so much data I loss track whenever a Lead gets to that part of the funnel if the Lead was created in the last month. For example, I loss a lot of valuable tracking data from the 1st to the 5th every month.

So that´s the matter, I want to filter that Checkbox as easy as I filter the created Leads by date, is there a way to do this? My best guest would be to create a datetime Field and relationate it with that Checkbox, so it get´s triggered every time an user updates the lead to that checkbox marked. What do you guys think about this? I want to read some feedback!

Have a nice day and thank´s for reading!