Update WHATSNEW

docs/WHATSNEW

@@ -1,5 +1,30 @@
 CHANGES TO REMIND
 
+* Version 3.1 Patch 18 - TBD
+
+- IMPROVEMENT: Add introspection functions trigback(), trigdelta(),
+  trigduration(), trigfrom(), trigpriority(), trigrep(),
+  trigscanfrom(), trigtimedelta(), trigtimerep(), and triguntil().  See
+  man page for details; thanks to Tim Chase for the suggestion.
+
+- CHANGE: Modify addition so that previously-illegal combinations
+  TIME + TIME, TIME + DATETIME and DATETIME + TIME are now allowed.  If
+  t1 and t2 are expressions of type TIME and dt is an expression of
+  type DATETIME, then the following are now equivalent (before, the
+  expressions on the left-hand side would fail with a "Type mismatch"
+  error.)
+
+  t1 + t2     ==     t1                + coerce("INT", t2)
+  dt + t2     ==     dt                + coerce("INT", t2)
+  t1 + dt     ==     coerce("INT", t1) + dt
+
+- DOC FIX: Document previously-undocumented $MaxStringLen system variable
+
+- BUG FIX: Catch potential date overflow in slide() function
+
+- BUG FIX: Fix compile error when compiling Romanian version; eliminate
+  compiler warning when compiling non-English versions.
+
 * Version 3.1 Patch 17 - 2019-11-15
 
 - IMPROVEMENT: Add "Extra Remind Options" setting to TkRemind.