Sex 15 days before period. Safe?

No. This is midcycle and when you may be most fertile. If you don't want to get pregnant, don't have sex at this time. See this link for more information: http://www.Plannedparenthood.Org/health-topics/birth-control/symptothermal-method-22142.Htm.
Safe for what ? Conception can occur on any day of a cycle including during menses.It is more common at some times than others but if it matters, use countermeasures.
Less likely but... Unprotected sex in a fertile woman with a fertile man has a possibility of conception, but the greatest chance is right around the time of ovulation. This was probably a bit too early, but there is a chance. If you miss a period - do a test. If you do not want to conceive, use protection always. Best!