I presume you're referring to the date shown here:
That is the date the member joined the community. The date a member creates their giffgaff account, registers their SIM, starts their first goodybag and visits the community for the first time can all be different.
Example: A prospective new member visits giffgaff on 10th of April. They order a SIM and pre-select or prepay for a goodybag. The SIM arrives 2 days later and they activate the SIM (goodybag starts). Two days later they come on the community to have a nose around or ask a question
Account creation date (date the goodybag was pre-selected/prepaid): 10 April
SIM activation date: 12th April (also goodybag start date)
Community start date: 14th April
I do not work for giffgaff, so cannot check anything on your account. This is a public community - please do not post any personal info.