Current version of software is 4.3 / DB:

My client birth-date is reverting to the year 1900

With several users in Australia and one in Canada (and a problem that was quite common in Europe 7-8 years ago).
That DOB problem is usually fixed by changing your windows short date format settings to mm/dd/yy.  Go in to the Windows date settings and tell it very specifically what you want the "short" date to be. 
If this does not resolve the problem submit a support ticket and include the following information:
The version of Windows you are using?  
Is it happening on existing clients or new clients?
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  • 07-Jun-2017