my $birthday = '2000-01-02'; # yyyy-mm-dd
use Date::Calc qw/Delta_Days Today Add_Delta_YMD/;
my @date = split /-/, $birthday;
$date[0] = (Today())[0];  # set the year to current year
print Delta_Days( Add_Delta_YMD(@date,-1,0,0), @date );  # days since last one
print Delta_Days( @date,  Add_Delta_YMD(@date,1,0,0) );  # days until next one