How do I calculate years between transactions?

[kelSAS]

Hi, 

I have firm data with IPO. It looks like 

year firmid IPO

1990 abc 1

1995 abc 1

1997 def 1

2000 def 1

2000 ghi 1

2002 jkl 1

I want to calculate the years between IPOs if a firm has more than 1 IPO. 

For example, 

year firmid IPO duration

1990 abc 1 0

1995 abc 1 5 (1995-1990)

1997 def 1 0

2000 def 1 3 (2000-1997)

2000 ghi 1 0

2002 jkl 1 0

Thank you for your help in advance. 

[Reeza]

Look at the DIF function