Activity End date sync input validation

I have a customer syncing Timeline Activities to Salesforce and we just recently made the field Activity End Date a default field. However this field CANNOT be a negative value otherwise it will not sync to Salesforce. 
Examples:

3 activities were created; 
Activity Date for all of them 1:00PM Central
Activity End Dates of 1:00PM & 12:59PM. 

The 1:00PM synced, the 12:59PM didn't, just confirming that functionality there. 

This is working as designed but the customer brought up a great point in that it leaves a lot of room for error.

The Activity Date is auto-populated for the point in time of when you click "Log a Meeting". Activity End Date field doesn't auto-populate, and they will manually enter that information. So consider the following scenario:

- I had an onsite meeting at 1PM yesterday with a customer and travelled home overnight. 
- I go to log my Activity today and click "Log a Meeting" 
- The Activity date auto-populates to today at 8:09AM and either I assume that's just the date I recorded this activity or I just don't notice it. 
- I see the Activity End Date is blank (and required) so I add yesterday at 1:00PM because that is when I actually met with the customer. 
- I click "Log" 
- The activity doesn't sync to SF

Could we potentially address this by making the end date auto populate along with the Activity date and in that case it will still make sure its syncs rather than a negative value enter and the sync break? 

Any other solution is also optional, I would like to know how we could address this.