diff --git a/inst/+tblish/+internal/+chrono/+tzinfo/TzInfo.m b/inst/+tblish/+internal/+chrono/+tzinfo/TzInfo.m
index 6ee02243..6d2a31b2 100644
--- a/inst/+tblish/+internal/+chrono/+tzinfo/TzInfo.m
+++ b/inst/+tblish/+internal/+chrono/+tzinfo/TzInfo.m
@@ -28,7 +28,9 @@
   properties
     id
     formatId
-    # These fields are all parallel
+    # These fields are all parallel.
+    # The transitions and transitionslocal are int64s holding (I think) Unix times.
+    # The *Datenum suffixed ones are doubles holding datenums.
     transitions
     transitionsLocal
     transitionsDatenum
@@ -154,7 +156,7 @@ function prettyprint (this)
       if (ismember (this.id, tblish.internal.chrono.tzinfo.TzInfo.utcZoneAliases))
         # Have to special-case this because it relies on POSIX zone rules, which
         # are not implemented yet
-        offsets = zeros (size (dnum));
+        out = dnum;
       else
         # The time zone record is identified by finding the last record whose start
         # time is less than or equal to the local time.
@@ -163,8 +165,11 @@ function prettyprint (this)
         # the right transition point. (It has to be a *modified* bin search
         # because the local times for transitions are not necessarily monotonic increasing.
         # I think.)
+        #
+        # FIXME: Handle NaN inputs.
         tf = false (size (dnum));
         loc = NaN (size (dnum));
+        last_transition = this.transitionsLocalDatenum(end);
         for i_dnum = 1:numel(dnum)
           d = dnum(i_dnum);
           ix = find(this.transitionsLocalDatenum <= d, 1, "last");
@@ -175,17 +180,22 @@ function prettyprint (this)
         endfor
         # [tf,loc] = tblish.internal.chrono.algo.binsearch (dnum, this.transitionsLocalDatenum);
         ix = loc;
-        tfOutOfRange = isnan(ix) | ix == numel (this.transitions);
+        tfOutOfRange = (! isnan (dnum)) & (isnan(ix) | ix == numel (this.transitions));
+        tfToConvert = (! isnan (dnum)) & ! tfOutOfRange;
+
+        out = dnum;
         # In-range dates take their period's gmt offset
-        offsets = NaN (size (dnum));
-        offsets(!tfOutOfRange) = this.ttinfos.gmtoffDatenum(this.timeTypes(ix(!tfOutOfRange)));
+        if any (tfToConvert(:))
+          offsets = this.ttinfos.gmtoffDatenum(this.timeTypes(ix(tfToConvert)));
+          out(tfToConvert) = dnum(tfToConvert) - offsets;
+        endif
         # Out-of-range dates are handled by the POSIX look-ahead zone
         if (any (tfOutOfRange(:)))
-          # TODO: Implement this
-          error ('POSIX zone rules are unimplemented');
+          # FIXME: Implement this
+          error ('datetime: POSIX zone rules are required to convert out-of-tzdb-range dates, but are unimplemented');
         endif
+        # Anything left over should be NaNs; no change needed.
       endif
-      out = dnum - offsets;
     endfunction
 
     function out = gmtToLocaltime (this, dnum)
@@ -227,6 +237,10 @@ function displayCommonInfo (this)
       fprintf ('  Version %s (%s time values)\n', this.formatId, time_size);
       fprintf ('  %d transitions, %d ttinfos, %d leap times\n', ...
         numel (this.transitions), numel (this.ttinfos.gmtoff), numel (this.leapTimes));
+      if (! isempty (this.transitions))
+        fprintf ('    First transition: %s   Last transition: %s  (UTC)\n', ...
+          datestr (this.transitionsDatenum(1)), datestr (this.transitionsDatenum(end)));
+      endif
       fprintf ('  %d is_stds, %d is_gmts\n', ...
         numel (this.isStd), numel (this.isGmt));
       if (! isempty (this.goingForwardPosixZone))
diff --git a/inst/datetime.m b/inst/datetime.m
index 3190c1e8..1f538a3f 100644
--- a/inst/datetime.m
+++ b/inst/datetime.m
@@ -69,7 +69,7 @@
   endproperties
 
   properties (Access = private)
-    # The underlying datenum values, always in UTC
+    # The underlying datenum values, always in UTC when zoned, or pseudo-UTC when unzoned.
     dnums = NaN % planar
   endproperties
   properties
@@ -238,13 +238,17 @@
       endswitch
 
       # Construct
+      # FIXME: The set.TimeZone here invokes conversion before dnums is set, which
+      # hits a NaN-not-supported case
       if (! isempty (timeZone))
         this.TimeZone = timeZone;
         if (! isequal (timeZone, 'UTC'))
           dnums = datetime.convertDatenumTimeZone(dnums, timeZone, 'UTC');
         endif
+        this.dnums = dnums;
+      else
+        this.dnums = dnums;
       endif
-      this.dnums = dnums;
       if (isfield (opts, 'Format'))
         this.Format = opts.Format;
       endif
@@ -392,6 +396,9 @@
         error ('Undefined TimeZone: %s', x);
       endif
       if (isempty (this.TimeZone) && ! isempty (x))
+        # Converting an unzoned date to a zoned one declares that the wall time represented by
+        # the unzoned dnums is a local time. Zoned datetimes store their internal dnums in UTC,
+        # so we need to convert *from* the new time zone to UTC for the internal representation.
         this.dnums = datetime.convertDatenumTimeZone (this.dnums, x, 'UTC');
       elseif (! isempty (this.TimeZone) && isempty (x))
         this.dnums = datetime.convertDatenumTimeZone (this.dnums, 'UTC', this.TimeZone);
@@ -1598,10 +1605,10 @@ function disp (this)
 
 %!test datetime;
 %!test datetime ('2011-03-07');
-%!xtest <unimplemented timezone support> datetime ('2011-03-07 12:34:56', 'TimeZone', 'America/New_York');
+%!test datetime ('2011-03-07 12:34:56', 'TimeZone', 'America/New_York');
 %!test
 %!  d = datetime;
 %!  d.TimeZone = 'America/New_York';
 %!  d2 = d;
 %!  d2.TimeZone = 'America/Chicago';
-%!  assert (abs(d.dnums - d2.dnums), (1/24), .0001)
+%!  assert (abs (datenum (d) - datenum (d2)), (1/24), .0001)