Update release notes.

docs/WHATSNEW

@@ -1,5 +1,63 @@
 CHANGES TO REMIND
 
+* VERSION 5.4 Patch 0 - 2025-??-??
+
+- MAJOR IMPROVEMENT: remind: Track which expressions and variables
+  Remind can prove to its satisfaction are "constant" - that is, that
+  will always have the same value on any Remind run as long as the
+  files are not edited.  This allows "Purge Mode" to purge more
+  reminders than before.
+
+  See the new remind(1) man page section "NON-CONSTANT EXPRESSIONS"
+
+- MINOR NEW FEATURE: remind: add the "nonconst" built-in function (to
+  force an expression to be interpreted as non-constant) and the
+  "isconst" function to test if an expression is constant.
+
+- MINOR NEW FEATURE: remind: Add eval() built-in function that takes a
+  string argument and evaluates it as an expression.
+
+- MINOR NEW FEATURE: remind: Add the trigbase() function and
+  corresponding $Tb system variable.  See the remind(1) man page for
+  details.
+
+- MINOR IMPROVEMENT: remind: Make $Tu a synonym for triguntil().
+
+- MINOR IMPROVEMENT: remind: If coerce() fails, issue a more detailed
+  error message.
+
+- MINOR IMPROVEMENT: remind: Using Hush mode "-h" with purge mode "-j"
+  suppresses the normal purge-mode diagnostics about reminders that
+  might not be purged.  Instead, only the "#!P: Expired" purge-mode
+  lines are emitted.
+
+- NEW FEATURE: remind: Add the catch() built-in function.  This lets
+  you catch errors in expression evaluation.  For example:
+  catch(2/0, "oops") returns "oops".  The catcherr() function returns
+  the error message from the last catch() expression that failed, in
+  this case "Division by zero".
+
+- IMPROVEMENT: remind: Remind can handle dates that require UTC to
+  Local conversion (and vice-versa) for years greater than 2038, even
+  on systems with a 32-bit time_t.
+
+- IMPROVEMENT: rem2html: Include an anchor <a id="d$day"> in the HTML
+  calendar for each day box, where $day is the day number.
+
+- MINOR IMPROVEMENT: remind: Allow the argument to monnum() to be a
+  string, in which case it's parsed as a month name and the
+  corresponding month number returned.
+
+- MINOR IMPROVEMENT: remind: Allow the argument to wkdaynum() to be a
+  string, in which case it's parsed as a weekday name and the
+  corresponding weekday number returned.
+
+- MINOR IMPROVEMENT: remind: Make the expression parser optimize away
+  "- INT_CONSTANT" and "! INT_CONSTANT".
+
+- BUG FIX: tests: Don't use the obsolete "tail +2" syntax in test-rem.
+  Fix courtesy of Dick Marinus.
+
 * VERSION 5.3 Patch 7 - 2025-05-06
 
 - IMPROVEMENT: examples/astro: Make moonrise/moonset appear after