Expiring data, 31 day that turn into 10 days

Morning, every Month I buy R500 worth of airtime Telkom mobile and I buy monthly bundle, my b Problem is that when you purchase this bundles it say buy for 31 days or 60 days, if I buy for 31 days,after loading them it tells you it will expire in 10 days this is now being happening for a while, why are they not just say your 5 gig will last for 10 day including the night surf. This is really unacceptable for this network, I've been with them for years now, I just feel like moving to onother network is this is clearly killing me