To elaborate on expiration dates, the date most distant in the future is the one which will apply to ALL of your funds.

For example if you add $15 on May 1, ALL of your funds will expire after 30 days. If you do not add additional funds within 30 days, ALL funds in your account(including those which have not been used) will disappear at the end of 30 days.

IF HOWEVER, you ADD additional funds prior to the end of that 30 days, ALL funds in your account will have a NEW expiration date. Say you add $100 prior to the end of those 30 days, May 15 for example, then ALL of the funds in your account will expire at the end of May 14 in 1 year including any funds still in your account from previous refill cards such as that $15 you added on May 1.