Visa expiration and 15 days stay

when submit visa japan , attach air ticket 21 apr arrive landing japan and 27 apr depart from japan to other country,
when collect visa , issue date 23 jan and expire date 23 Apr . for stay 15 days.
my question is :
it is before 23 Apr I must out from japan?
or it is before 23 apr landing japan , i can stay 15 days after landing date.
a bit confuse about expiry date of visa .
thankyou

The dates that your arrival must be between.

15 days starts when you arrive.

The visa (what you use to “enter” Japan) is valid until April 23, three months from the date of issuance.
So you need to “enter” Japan while the visa is valid, so before April 23.
And from that date you are allowed to stay for max. 15 days.