looking for After Effects expression that lets me loop numbers counting up for day, month, year

Try this Expression,, and don't forget to add Time slider,

monthnames=["01", "02", "03", "04", "05", "06", "07", "08", "09", "10", "11", "12"];

Speed=effect("Time")("Slider");

date_1=new Date(1918,11,5); // watchout month 0= january, 11=:december !
milli_1=date_1.getTime();

date_2=new Date(1923,10,15);
milli_2=date_2.getTime();

t=linear(time,0,Speed,milli_1,milli_2);

date_3=new Date(t);

date_3.getFullYear() + "-" + monthnames[date_3.getMonth()] + "-" + date_3.getDate() ;

Project file:

https://www.dropbox.com/s/5nkqjd2qm0gl7zz/Expression-Time.aep?dl=0

From Dan Ebberts:

startDate = "Dec 5 1918";
endDate = "Nov 15 1923";
rate = 200; // counts per second

d1 = new Date(startDate);
d2 = new Date(endDate);
msPerDay = 24*60*60*1000;
t = Math.floor((time-inPoint)*rate);
ms = Math.min(d1.getTime() + t*msPerDay,d2);
d = new Date(ms);
dy = "" + d.getDate();
mo = "" + (d.getMonth()+1);
yr = "" + d.getFullYear();
if (dy.length < 2) dy = "0" + dy;
if (mo.length < 2) mo = "0" + mo;
yr + "-" + mo + "-" + dy

You have to do the math to figure out how many days that is then set the rate to the value that will give you an animation that lasts the right amount of time. 200 may be too fast, but then again 200 may be too slow.

Personally, I think this is going to look really bad. All of those changing numbers. I would use 3 instances of CC Cylinder and put 30 days on one of them, 12 months on another, and seven years on the other. Unless your timeline is hours long the days are going to be nothing but a blur anyway so there is no need to worry about 28 days in February or 31 in January. The months don't need to line up with the days if they are a blur. Then you could just keyframe the days to spin quickly, the months to spin a little slower so you could see them start to speed up and then come to a stop, and then set 7 keyframes for the dates. This would look like an odometer spinning and be very easy to set up. 

If you really want numbers then set keyframes for 3 animation sliders, one for the year, one for the day and one for the month. Then start them changing and add some Pixel Motion Blur or CC Force Motion Blur to try and blend the text together as the days change. Trying to do this literally is not really a good design idea unless the animation is going to be going by so slowly that the average viewer can tell the if the days and months go out of sync.

If you want to try the CC cylinder idea it just takes 3 text layers in pre-comps with CC Cylinder applied to each, then lined up.