From this answer a modern day is 1,7ms longer than a century ago. The data is taken from wikipedia, but it is sourced to this book.
Asuming the slowing down on earth's rotation has been constant since Cambrian, and that the sideral year duration has been constant too, it can be aproximated with a script:
#Constants
yearsSinceCambrian = 541000000
slowDownSecondsDayEachYear = 0.000017
daysPerYearPresent = 365.25
secondsDayPresent = 86400
#Initiallizate the variable
daysSinceCambrianExplosion = 0
#Calculate the number of days of each year
def sumYearDays(year):
secondsDay = secondsDayPresent - slowDownSecondsDayEachYear*year
hoursDay = secondsDay/3600
daysYear = daysPerYearPresent*24/hoursDay
return daysYear
#Sum all days from Cambrian Explosion to year 0
for i in range (0,yearsSinceCambrian):
daysSinceCambrianExplosion += sumYearDays(i)
#Sum days since year zero
daysSinceYearZero = 737875
daysSinceCambrianExplosion += daysSinceYearZero
#Print the result
print (str(daysSinceCambrianExplosion) + " days approximately since life explosion.")
Output
208929424039.7131 days approximately since life explosion.
Said $2.09 \cdot 10^{11}$ days.
I know the moon has moved away since Cambrian, so I assume as a constant wich is not. There are some extra days not summed.
I calculated the days the year had 510 my ago on my script and I got 406 days. In this publication from NASA 510 million years ago the year had 424 days, apparently; 18 more days. I think they know it because of fossils. I must say if paleontologists are rigth, if you examine the table published the slowing down ratio has not been constant. Ocean basins and tectonic should influence. So I can't be sure to use 1,7ms/century is accurated for the hole serie. In the past it migth have been 1,8 or 1,9 ms/century who knows.
To estimate the epsylon of the script, you can multiply the half of the 18 extra days on Cambrian for the total years of the serie.
$9 * 541000000 = 4.9*10^9$.
That would give a total of $2.14 \cdot 10^{11}$ days. This is not very far from what the script calculates, so you can use it to calculate from other events, knowing it is an aproximation.
The exact number of days can't be calculated, but there is not neither a exact known day when Phanerozoic started. Nobody can say the Phanerozoic started a 1st january 541002020 years ago, as assumed in the script.
I will say so @samcarter_is_at_topanswers.xyz calcs should be correct and approximately $2.1 \cdot 10^{11}$ days ago.