SQL Server 2012

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create table "Test"(
  "id" int,
  "number" varchar(15),
  "date" date)
insert into "Test" ("id","number","date") values
( 7387,'0101008461','2016-04-07 17:31:54.690'),
(77109,'0101008461','2018-01-25 11:11:19.720'),
(61009,'0105003182','2017-09-08 11:25:28.403'),
(50859,'010501748932','2017-06-23 11:52:28.237'),
(50998,'010501748932','2017-06-26 11:30:13.357'),
( 9978,'010501748932','2016-05-06 12:43:41.813');
6 rows affected

select * from "Test"


id
number
date


7387
0101008461
07/04/2016 00:00:00

77109
0101008461
25/01/2018 00:00:00

61009
0105003182
08/09/2017 00:00:00

50859
010501748932
23/06/2017 00:00:00

50998
010501748932
26/06/2017 00:00:00

9978
010501748932
06/05/2016 00:00:00


select
  t01."id",
  t01."number",
  t01."date" "date_current",
  t03."date" "date_previous",
  datediff(day,t03."date",t01."date") "days"
from "Test" t01
   left join "Test" t03 
   on 
     t03."number" = t01."number"
     and t03."date" = 
    (
     select
       max(t02."date") "date"
     from "Test" t02
     where
       t02."number" = t01."number"
       and t02."date" < t01."date"
    )


id
number
date_current
date_previous
days


7387
0101008461
07/04/2016 00:00:00
null
null

77109
0101008461
25/01/2018 00:00:00
07/04/2016 00:00:00
658

61009
0105003182
08/09/2017 00:00:00
null
null

50859
010501748932
23/06/2017 00:00:00
06/05/2016 00:00:00
413

50998
010501748932
26/06/2017 00:00:00
23/06/2017 00:00:00
3

9978
010501748932
06/05/2016 00:00:00
null
null

id	number	date
7387	0101008461	07/04/2016 00:00:00
77109	0101008461	25/01/2018 00:00:00
61009	0105003182	08/09/2017 00:00:00
50859	010501748932	23/06/2017 00:00:00
50998	010501748932	26/06/2017 00:00:00
9978	010501748932	06/05/2016 00:00:00