Flag date overlaps greater than 28 days

[Tomcaty]

I have the following data set that I would like to use to calculate overlapping days between two drugs. If the overlap period is  >=28 days then flag the ID. 

Source: 

ID	Drug1	d1start_dt	d1End_dt	Drug2	d2start_dt	d2End_dt
1	A	12-Jul-17	10-Oct-17	B	.	.
2	B	1-Jan-00	30-Jan-02	A	10-Apr-01	30-Nov-04
3	C	22-Aug-18	20-Nov-18	C	.	.
4	A	12-Sep-18	20-Dec-18	A	7-Dec-18	16-Apr-19
5	C	4-Jun-15	13-Mar-18	C	.	.
6	D	15-Jun-18	13-Sep-18	B	29-Jul-14	27-Aug-16
7	A	8-Jun-16	6-Sep-16	D	13-Aug-15	3-Aug-16
8	G	28-Mar-16	26-Jun-16	E	.	.
9	H	13-Apr-17	18-Jul-18	H	21-Jan-18	10-Jun-18
10	B	20-Feb-17	4-Jun-17	C	.	.

Result:

ID	Drug1	d1start_dt	d1End_dt	Drug2	d2start_dt	d2End_dt	Overlap days	Flag
1	A	12-Jul-17	10-Oct-17	B	.	.	0	0
2	B	1-Jan-00	30-Jan-02	A	10-Apr-01	30-Nov-04	295	1
3	C	22-Aug-18	20-Nov-18	C	.	.	0	0
4	A	12-Sep-18	20-Dec-18	A	7-Dec-18	16-Apr-19	13	0
5	C	4-Jun-15	13-Mar-18	C	.	.	0	0
6	D	15-Jun-18	13-Sep-18	B	29-Jul-14	27-Aug-16	0	0
7	A	8-Jun-16	6-Sep-16	D	13-Aug-15	3-Aug-16	390	1
8	G	28-Mar-16	26-Jun-16	E	.	.	0	0
9	H	13-Apr-17	18-Jul-18	H	21-Jan-18	10-Jun-18	140	1
10	B	20-Feb-17	4-Jun-17	C	.	.	0	0

[PaigeMiller]

The overlap seems to be (if I am understanding you properly) when d2start_dt < d1End_dt (although it would be nice if you had stated that directly to eliminate my possible misunderstanding). If that's the case, then the relatively simple test of d1End_dt - d2start_dt >=28 results in a flag.

if d1End_dt - d2start_dt >= 28 then flag=1;
else flag=0;

This assumes that all of your dates are valid SAS dates, and not character strings.