To reproduce the problem:

1. Create a customfield type "Date Time" as Date Of Birth
2. Try to create a issue with customfield Date of Birth having 1970/1/1 value
3. Create another issue with customfield Date of Birth hainvg 1965/1/1 value
4. Try to search for issues using JQL (JIRA Advance Searching) : "date of birth" < "1980/01/01"

It will only result issue with customfield Date of Birth having 1970/1/1 value