The current RTC I'm using, the ISL1208 does a good job in all departments except the time-keeping current consumption. 400 nA is a bit high because I've seen other RTCs with standby current half of that. But none of them had month alarm feature which is a crucial requirement for our application. So searching for an RTC with lesser standby current I found two : the PCF85263A from NXP and AM0805 from Ambiq.
PCF85263A has all the features we want; month alarm (it has two alarms), I2C, wide operating voltage (0.9 to 5.5V), interrupt output pin, and time-keeping current of only 280 nA typical. So that's better than the ISL1208. ISL1208 in the current design can be replaced with PCF85263A with a little modification in the code. This will reduce the system sleeping current to (600 nA - 120 nA) = 480 nA.
Isn't that enough? Well then, we have the ultimatum - the Ambiq AM0805 - majestic features on a small all-in-one package. Its time-keeping current is insanely low - 14 nA at 3V. it has integrated time crystal, month alarm, countdown timer, interrupt output, external interrupt monitor, I2C etc. But unfortunately this IC is not available in India. I'll have to buy this from any foreign distributors. That's the only bummer! But if we get it, we could reduce the system current consumption to just the microcontroller sleeping current only which will improve the expected operating time to about 73 years.