Depends on who your asking.All my freinds, family give it to me willingly.
Coworkers, teachers and aquaintances are more trickier.
Facebook is a goodway. Both directly and Indirectly. Of course, you might encouter private accounts or in my case profiles that are practically invisable.
There are still ways around it, but at that point theres probably and easier way and you don't want to "hack it". If its only for there birthtime.
Other times you'll get lucky or find out by accident. For example I always wondered what one of my coworkers B-Day was.
I was at work at my desk "cleaning" out my email requests on my outlook. After the 5th email I find an email that says "xxxx's Lunch Party Cancelled".
Automatically, I learn when xxxxx birthday is.
Or there might be an office calander shared in your workgroup, with event,plans,goals, etc months in advance including coworker birthdays.
Worst case scenario you'll have to be creative in finding information if you know the person will react to you negativly if your asking there birthday for astrological reasons.
Finding someons birthday is possible, heck google might even help you depending on what your searching.
Birthtime is more or less imposible.
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Capircorn Rising
Gemini Sun
Aries Moon
Mercury in Gemini
venus in Tarrus
Mars Cancer