I agree with you, in the next century(by meaning this , 21st century), difference will grown to 14 days, correct? So, in the 19th century her b-day was on June 26, in 20th she celebrated on June 27. But June 14 (old Calendar hasn't moved up to 15, or 16th, since it is impossible). Russia accepted New Calendar in 1919, so I presume, her real b-day was on June 14 (to her death,in 1918, complicated, I know, but still true).