how many days has it been since the first day of the world?
Assuming we're talking about the beginning of time as we know it from the big bang. Take 13.8 billion years ago (13,800,000,000) and assume the calendar still applies, multiply by 3.875e+11 which gives you 5,037,000,000,000 (five trillion, thirty-seven billion) days since the beginning of time since the big bang.