How to catch the time stamp of three days ago?

Today it is 10 shock 19

echo strtotime(date("Y-m-d", time()));

output 1539921600
is 12:0:0 on 2018-10-19

three days ago, it was 10 shock 16

echo strtotime(date("Y-m-d", strtotime("-3 days")));

output 1539662400
is 12:0:0 on 2018-10-16
seems to be twelve more hours (43200)
what I want to catch is

Today, 2018-10-19, 0:0:0 is the time after 23:59:59
and
three days ago 0:0:0 on 2018-10-16 is after 23:59:59
how to take this time

seems strange

I think 43200 is fine

echo strtotime(date("Y-m-d 0:0:0", time()-43200));

but it became 1539835200, that is, 12:0:0 on 2018-10-18.
the addition was not correct either. It became 1539921600, that is, 12:0:0 on 2018-10-19

how can this catch be (on Sunday) 0:0:0 on 2018-10-19 and (three days ago) 2018-10-16 000 040 0?