I think Lenkov said it's actually March 10th, even though the ep aired on March 19th. It's based on the birthdate given for Steve when his arrest warrant is on the computer monitor/other big screen in the S1 finale ep (in the scene where HPD Sgt. Duke Lukela is warning Chin, by phone, that HPD is coming to Five-0 HQ to arrest Steve for murdering Laura Hills; Duke's looking at Steve's arrest warrant on his computer as he's talking to Chin). That date is apparently March 10, 1977.