Maxim's RTC like DS1302,DS1399,DS1340 have back-up battery feature , allowing RTC to continue work during VCC fail time. But sometime customer find after VCC fail and recover, they can not read RTC time correctly. This is possible caused by the falling rate of VCC. VCC falls under lowest supply voltage before the internal power rail switches to Vbat.
These parts requires the falling rate of VCC being 0.27V/300us. （3.3V VCC）For 3V and 1.8V VCC, please refer to table below.
So to avoid power down between switching from VCC to Vbat, enough capacitor should be added at the VCC pin.