SQLite 3.27

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

create table testTable (date_c timestamp);
✓
insert into testTable values ('1982-01-10T00:00:00Z');
✓
insert into testTable values ('1982-01-11T00:00:00Z');
✓
insert into testTable values ('1982-01-14T00:00:00Z');
✓
insert into testTable values ('1985-01-16T00:00:00Z');
✓
insert into testTable values ('1985-01-17T00:00:00Z');
✓
insert into testTable values ('1985-02-12T00:00:00Z');
✓
insert into testTable values ('1987-01-11T00:00:00Z');
✓
insert into testTable values ('1987-01-12T00:00:00Z');
✓
insert into testTable values ('1987-01-13T00:00:00Z');
✓
select min(date_before), count(date_c)+1, month, year from
(select strftime('%d',date_c) - lag(strftime('%d',date_c)) over (order by date_c) diff
       , strftime('%d',date_c) day
       , lag(strftime('%d', date_c)) over (order by date_c) day_before
       , strftime('%m', date_c) month
       ,  lag(strftime('%m', date_c)) over (order by date_c) m_before              
       , strftime('%Y', date_c) year
       , lag(strftime('%Y', date_c)) over (order by date_c) y_before
       , date_c
       , lag(date_c) over (order by date_c) date_before
from testTable
order by date_c)
where diff = 1
and month = m_before
and year = y_before
group by  month, year;


min(date_before)
count(date_c)+1
month
year


1982-01-10T00:00:00Z
2
01
1982

1985-01-16T00:00:00Z
2
01
1985

1987-01-11T00:00:00Z
3
01
1987

min(date_before)	count(date_c)+1	month	year
1982-01-10T00:00:00Z	2	01	1982
1985-01-16T00:00:00Z	2	01	1985
1987-01-11T00:00:00Z	3	01	1987