The following relates to our current implementation of GlassPass timecards which can be purchased from this page: 

Store purchases:

At this time timecards are immediately applied to your account after purchase and remain in affect for their duration after which they do not automatically renew.

IndieGoGo backer keys:

If you backed us on IndieGoGo your timecard key can be applied to your account at the time of your choosing so you may use it now, or wait and begin using it at any point in the future. 

IMPORTANT: If you have an active Timecard on your account redeeming an additional key or purchasing an additional timecard WILL NOT STACK. For example if you purchase two 1 month GlassPass timecards you will not receive 2 months of time on your account, at the end of one month they will both expire. We will be resolving this unexpected behavior in the future, if you inadvertently have redeemed multiple timecards on your account contact us and we will take care of you