How to use Lookup Match or Index to find date of 1st sale to Customer?

[studawg]

Hi Masters
I have a list of 15000+ Customers with monthly sales since 2011.
What is the correct formula to identify the month in which each Customer 1st bought (i.e. when their sales are > 0)?
In the sample file attached I have manually selected the date from the 1st purchase (yellow cell), but need to automate the process for so many rows.
Thanks

 

[Chihiro]

Something like below:
in BP5:
=INDEX($B$1:$BO$1,,SMALL(IF(B5:BO5>0,COLUMN($B$1:$BO$1)-1),1))
Confirmed as Array (CSE)
Copy down.

 

[Chihiro]

Or non array version.
=INDEX($B$1:$BO$1,MATCH(TRUE,INDEX($B5:$BO5>0,0),0))

 

[bosco_yip]

Another non-array formula option.

In BP5, formula copy down:

=INDEX(B$1:BO$1,MMULT(MATCH(,0/(B5:BO5>0),0),1))

Regards
Bosco

 

[studawg]

Another non-array formula option.

In BP5, formula copy down:

=INDEX(B$1:BO$1,MMULT(MATCH(,0/(B5:BO5>0),0),1))

Regards
Bosco

THnaks guys both work great!