[Bumbis]

pawn Код:

new Dates[3][11];
new DateString[11]="04/02/1997";
split(DateString,Dates,'/');

new day = strval(Dates[0]);
new month = strval(Dates[1]);
new year = strval(Dates[2]);

if(day<1||day>31) SendClientMessage(playerid,0xFFFFFFFF,"Day can only be from 1 to 31");
if(month<1||month>12) SendClientMessage(playerid,0xFFFFFFFF,"Month can only be from 1 to 12");
if(year<1) SendClientMessage(playerid,0xFFFFFFFF,"Year can be less than 1");

It's a example how you can do it with dialogs or whatever you need. Just make DateString variable equal to inputtext.
Not tested, but it should work.