Using index match offset to find first date that amounts drop to zero

[RobTech1]

Hi,

Trying to build a formula that returns the first date that a series of amounts ceases, ie drops to zero, using an index on date range, and attempting to match where amount is zero and amount of month prior is >0.

Importantly, the series of payments may not begin in first month, so no value in just finding the first zero amount...

My formula currently is CSE but returning #n/a:

{=INDEX(C1:F1,,MATCH(TRUE,(C2:F2=0)*(OFFSET(C2:F2,0,-1)>0),0))}

Cells B1..F1 are a range of 5 dates from 1 Jan to 1 May

Cells B2..F2 are amounts 0,0,500,500,0

Month	1-Jan	1-Feb	1-Mar	1-Apr	1-May		Drop off date
PX $	0	0	500	500	0		#N/A

In the above case formula should be returning "1 May", as when series first dropped to zero.

Many thanks in advance...

[#2 · Accepted solution]

@RobTech1 

In the attached file, the formula in H2 is: 

=INDEX(B1:F1,
MATCH(LOOKUP(2,1/(B2:F2>0),B1:F1),
B1:F1,0)+1)

[#3]

Thanks @Twifoo - works perfectly, cheers.

[#4]

@RobTech1 

You're very much welcome!