make dates rendered server-side & consistently in YYYY-MM-DD format

client-side time zone adjustment still persists---the server renders them out in UTC, but the client will adjust the date to its timezone during loading. 
this shouldn't cause any layout shifting because we use `font-variant-numeric: tabular-nums` (though Recursive seems to use tabular numbers either way.)

crates/treehouse/src/html/tree.rs

@@ -1,6 +1,8 @@
 use std::{borrow::Cow, fmt::Write};
 
+use chrono::{DateTime, Utc};
 use treehouse_format::pull::BranchKind;
+use ulid::Ulid;
 
 use crate::{
     config::Config,
@@ -153,6 +155,15 @@ pub fn branch_to_html(
 
         s.push_str("<th-bb>");
         {
+            if let Some(date_time) = Ulid::from_string(&branch.attributes.id)
+                .ok()
+                .as_ref()
+                .map(Ulid::timestamp_ms)
+                .and_then(|ms| DateTime::from_timestamp_millis(ms as i64))
+            {
+                write!(s, "<th-bd>{}</th-bd>", date_time.format("%F")).unwrap();
+            }
+
             if let Content::ResolvedLink(file_id) = &branch.attributes.content {
                 let path = treehouse.tree_path(*file_id).expect(".tree file expected");
                 write!(

static/css/tree.css

@@ -318,9 +318,10 @@ th-bb {
 }
 
 /* Style branch dates to be smaller and less noticable. */
-th-bb .branch-date {
+th-bd {
     opacity: 50%;
     font-size: 0.8em;
+    font-variant-numeric: tabular-nums;
 }
 
 /* Hide branch dates on very small displays. No clue how to fix this just yet. */

static/js/tree.js

@@ -18,6 +18,10 @@ function branchIsOpen(branchID) {
     return branchState[branchID];
 }
 
+function dateToString(date) {
+    return new Date(date - date.getTimezoneOffset() * 60000).toISOString().substring(0, 10);
+}
+
 export class Branch {
     static branchesByNamedID = new Map();
     static onAdded = [];
@@ -70,11 +74,10 @@ export class Branch {
         }
 
         if (ulid.isCanonicalUlid(this.namedID)) {
+            // Adjust dates to fit the user's time zone.
             let timestamp = ulid.getTimestamp(this.namedID);
-            let date = document.createElement("span");
-            date.classList.add("branch-date");
-            date.innerText = timestamp.toLocaleDateString();
-            this.buttonBar.insertBefore(date, this.buttonBar.firstChild);
+            let branchDate = this.buttonBar.getElementsByTagName("th-bd")[0];
+            branchDate.textContent = dateToString(timestamp);
         }
 
         for (let callback of Branch.onAdded) {