The suggested VB solution could be very slow because it involves generating each of the times separately.

Here are some other options:

Filter On Yesterday's Date/15 minutes by using a SQL Query in a SSIS Data Flow Source:

SELECT *

FROM tbl_Readings

WHERE TimeStamp = [Yesterday]

AND DATEDIFF(minute,[Yesterday],TimeStamp) % 15 = 0

Filter On 15 minutes by using a SSIS Conditional Split Expression

(which you sound most familiar with):

DATEDIFF( "Minute", [Yesterday], TimeStamp ) % 15 == 0

These give you times that are an exact multiple of 15 minutes from [Yesterday].

They may need to be modified to suit your situation.