receipt date
Find below the portion of AC 21 memo from INS. It clearly states if the application to adjust status is not adjudicated within 180 days from filing, then the allien can change job. Filing usually means the date INS received the application(ie receipt date).
The AC21 106(c) provides that the certification or Form I-140 approval of an EB immigrant petition shall remain valid when an allien changes jobs, if:
a) a Form I-485, application to adjust status,on the basis of the EB immigrant petition has been filed and remained unadjudicated for 180 days or more; and
b)the new job is in the same or similar occupational classification as the job for which the certification or approval was intially made.