Nested If Statements in Excel /

Stackoverflow has been really helpful on my Nested IF confusions! I thought I'd ask one more question to help solve my seemingly complete lack of understanding the logic flow in Nested If's.

Current formula used in Cell (P3) is =IF(ISBLANK(M3),TODAY()-L3,M3-L3)

It works just fine, except for then there is no data in Start Date (L3) and Finish Date (M3). When both cells are Blank it shows the number 43,091.

The logic that I need however is as follows.

If both start date (L3) and Finish Date (M3) are BLANK, Result an Empty Cell (P3).

If Start Date (L3) has a date, and Finish Date (M3) is BLANK, Count the number of days between Start Date (L3) and TODAY.

If Start Date (L3) and Finish Date (M3) both have Dates, count the number of days between Start Date (L3) and Finish Date (M3).

I hate to be a pest but i just cannot wrap my head around it and have struggled with this for probably 2 hours now. ANY Help is appreciated everyone! Thank you so much.

James