tp.date.weekday() that’s a new one 
!
And I also had a lot of fun with Templater, Periodic Notes, Calendar, some JS but mostly Moment.js (once someone told me on discord this could be done) yesterday while setting up my Daily/Weekly/Monthly and Yearly Notes 
.
I use mostly the tp.file.title variable so most of what I use Templater for (for now) relies on how I format my daily (weekly/monthly/yearly) notes .
The tp.file.title variable is very useful when using Calendar/Periodic Notes as it still allows me to create any daily notes (either in the past or future using Calendar) without having to fear potential mistakes/errors.
As the non-dev’ person that I am 
, who has no JS knowledge (last time I did a little bit of javascript was 20 years ago 
) avoiding mistakes/errors is quite important .
But here is what I’ve got so far :
For the total of days (as a number) in a month :
In a Daily Note with the title formatted as YYYY-MM-DD and using Moment.js .daysInMonth() :
<% moment(tp.file.title,'YYYY-MM-DD').daysInMonth() %>
Example : for the daily note 2021-04-14, the result will be 30
For the Start/End of a month :
In a Monthly Note with the title formatted as YYYY-MM using Moment.js .startOf() or .endOf() and the result formatted as ddd. DD/MM
<% moment(tp.file.title,'YYYY-MM').startOf('month').format('ddd. DD/MM') %>
Example : for the monthly note 2021-04, the result will be Thu. 01/04
<% moment(tp.file.title,'YYYY-MM').endOf('month').format('ddd. DD/MM') %>
Example : for the monthly note 2021-04, the result will be Fri. 30/04
For the number of days left in a month (using a daily notes as a starting point):
Daily Notes title formatted as YYYY-MM-DD using .endOf() and .diff()
<% moment(moment(tp.file.title,'YYYY-MM-DD').endOf('month')).diff(moment(tp.file.title,'YYYY-MM-DD'),'days') %>
This 
is literally just a subtraction .
In other words what it does is find the last day in the month from the date in the title of the daily notes (moment(tp.file.title,'YYYY-MM-DD').endOf('month')) as its own Moment() and then subtracts the days from the daily notes title “date” (.diff(moment(tp.file.title,'YYYY-MM-DD'),'days' )
So if you have a daily note titled 2021-04-14 the result will be 16 (as there are 30 days in April)
For getting a list of all days in a month (“pre-linked” if the daily note already exists):
In a Monthly Note with the title formatted as YYYY-MM
It took me a while to get this one, lots of trials and errors 
(first shared on Discord and its based on this Stack Overflow reply )
<%* const currentMonthDates = new Array(
moment(tp.file.title,'YYYY-MM').daysInMonth()).fill(null).map(
(x, i) => moment(tp.file.title,'YYYY-MM').startOf('month').add(i,'days').format("- [[[]YYYY-MM-DD[#]YYYY-MM-DD[|]DD/MM [⦗]ddd[.⦘]]]")).join('\r\n') %>
<%* tR += currentMonthDates %>
What this kind of does is create a “list” (array) from the start of the month and as long as there are days in the month (based on the note’s title). Then it gives each item in this list a specific format (.format("- [[[]YYYY-MM-DD[#]YYYY-MM-DD[|]DD/MM [⦗]ddd[.⦘]]]")).
In my case, once Templater does what it has to do : - [[YYYY-MM-DD#YYYY-MM-DD|DD/MM ⦗ddd.⦘]].
There are lots of brackets [] because this is how Moment.js escapes characters so it can correctly interpret the format I’m asking for .
Once the list is created and formatted, it gets rid of the comma , which originally separates each item in the list and replace it with a new line to get an actual unordered list .
The last part <%* tR += currentMonthDates %> is to display the constant created in the first part (following Templater Documentation) .
For the list of all months in a year (“pre-linked” if the monthly notes already exists):
In a Yearly Note with the title formatted as YYYY.
Source : this Stack Overflow reply .
<% Array.apply(0, Array(12)).map(function(_,i){return moment(tp.file.title,'YYYY').month(i).format("- [[[]YYYY-MM[#]YYYY-MM[|]MM - MMMM[]]]")}).join('\r\n') %>
It “kind of” works in the same way as the previous one : creates a list of the months in the year referenced by the title, apply a specific format and then get rid of the comma , in the list .
Note :
- This part
Array(12) is the length of the list (12 items)
- This part
moment(tp.file.title,'YYYY') is the starting point of the list (based on the note’s title)
To get the DateTime (Now()) in whatever language you want :
<% moment(tp.date.now("YYYY-MM-DD HH:MM")).locale('fr').format("LLLL") %>
Updated and simplified in the post below :
… I might find others .
I’m absolutely no JS expert (far from that ) so there might be better ways to get to the same results too !
Edit : Added some context, explanations (as best as I could , trying to make this post more understandable for non-dev’ people like myself ) and sources .
!
!
) and I may have something for you 
Caution : I’m still no Javascript expert 
(but after lots of tests, what I’m going to suggest seems to work 
I will give it some well focussed attention, to bring myself to understand it. Thank you again.
Then again, Obsidian’s File Explorer pane shows the last modified date if you hover the mouse over a file, and it could possibly be retrieved “live” using the Dataview plugin.