Prorations are based on the membership plan's daily rate. If you have the 'billing day' set to either the 1st or 15th of the month under Triib Settings -> Edit Affiliate Info, the system will automatically prorate any membership invoice when the START DATE is different than the billing day. Therefore, if your gym is set to 1st of the month billing and someone's start date is the 2nd, one day will be discounted from the invoice.
Scenario #1: $100/month membership.
$100 x 12 = $1200 (12 months per year, total amount paid in one year)
$1200 / 365 = $3.29 (Daily rate).
Scenario #2: $150/month membership with 20% discount every month.
$150 - 20% = $120/month
$120 x 12 = $1440
$1440 / 365 = $3.95 (Daily Rate)
You also have an option for having a specific Membership Plan override the 1st or 15th of the month billing period if you wish to offer a certain member(s) the option to pay on their specific start dates if their payment doesn't line up exactly with your billing structure.