Hello!
Long story sort, I wanted my watch face to not only show the current date, but also the previous and next date numbers to the left and right respectively (I can’t remember the real watch I saw this on, but I thought it was genius [if anyone knows the brand/model, I’d love to see it, I can’t find it again 
]), because I don’t know about other people, but I can never remember how many days the current month has… I’m a bit ashamed to admit it but there you go, some things simply do not stick in my memory 
With that said, here is the code I use in all 3 boxes (previous date, current date, next date respectively):
Prev. Date:
$#DM#==1&&(#Dd#-1)<1?31:$$#DM#==2&&(#Dd#-1)<1?31:$$#DM#==3&&#Dd#==1&&#DD#==61?29:$$#DM#==3&&(#Dd#-1)<1?28:$$#DM#==4&&(#Dd#-1)<1?31:$$#DM#==5&&(#Dd#-1)<1?30:$$#DM#==6&&(#Dd#-1)<1?31:$$#DM#==7&&(#Dd#-1)<1?30:$$#DM#==8&&(#Dd#-1)<1?31:$$#DM#==9&&(#Dd#-1)<1?31:$$#DM#==10&&(#Dd#-1)<1?30:$$#DM#==11&&(#Dd#-1)<1?31:$$#DM#==12&&(#Dd#-1)<1?30:(#Dd#-1)$
Current Date:
#Dd#
Next Date:
$(#Dd#+1)>#DIM#?1:(#Dd#+1)$
Obviously the big chunk of work cut out for people will be the Prev. Date expression it’s nothing fancy and not really complicated, but it’s meant to cut down on some grunt work.
Note the expression also accommodates the month of February both in and out of Leap Year (this 2020 for example [leap year], on the 1st day of March, the 3 date boxes will read: 29 - 1 - 2)
There might be a simpler way to do all of this, and if there is, by all means if anyone would like to share, it’d be much appreciated 
Otherwise… this unfancy mess works just fine
For anyone interested in how the expression works, and might not know, it’s simply:
If the current month is X (January, February, March, etc in number form where January is 1, and December is 12), and the current date - 1 is below 1, then show last month’s maximum known date. Repeat for all the months of the year
For February it’s looking at 2 conditions; the first one checks if the current month is March and the current date is 1 and the current Day of the Year is 61; if all of this is true, then it’s a leap year, and the number here should be 29. Otherwise, if the current month is March and the current date - 1 is below 1 (as with every other month), then 28.
Then at the very end of the expression (and what takes over in any other date not covered above), it’s simply Current Date - 1.
Gif Demo February 27th, 2020 (leap year) through March 2nd.
Sadly I can’t post the watch face I’m using this on, as it’s meant to be a tribute to a watch I simply can’t afford and I recreated the face for private use (along with all the extra features I wanted on it)
Enjoy!
The point I was trying to make was that your code would not work with the leading zero style of date even when changing all the 
