diff --git a/flopy4/mf6/codec/reader/grammar/basic.lark b/flopy4/mf6/codec/reader/grammar/basic.lark
index 63f15f8f..c67eb7ad 100644
--- a/flopy4/mf6/codec/reader/grammar/basic.lark
+++ b/flopy4/mf6/codec/reader/grammar/basic.lark
@@ -1,17 +1,20 @@
 start: [_NL*] (block [_NL*])*
 block: "begin"i block_name _NL _list "end"i block_name [_NL+]
-block_name: CNAME [NUMBER]
+block_name: CNAME [TOKEN]
 _list: line*
-line: item* _NL+
-item: word | NUMBER
-word: /[a-zA-Z0-9'().\/:;<=>?@_~+\[\\-]+/
+line: TOKEN* _NL+
+// Every whitespace-delimited token lexes as one terminal, and the
+// transformer decides whether it's a number. Separate NUMBER and word
+// terminals overlap, and the lexer takes the first that matches rather
+// than the longest, so e.g. an ISO datetime `1997-07-16T19:20:30` lexed as
+// `1997` and `-07-16T19:20:30`.
+TOKEN: /[a-zA-Z0-9'().\/:;<=>?@_~+\[\\-]+/
 COMMA: ","
 
 %import common.NEWLINE -> _NL
 %import common.WS_INLINE
 %import common.CNAME
 %import common.WORD
-%import common.NUMBER
 %import common.INT
 %import common.SH_COMMENT
 %import common._STRING_INNER
diff --git a/flopy4/mf6/codec/reader/grammar/typed.lark b/flopy4/mf6/codec/reader/grammar/typed.lark
index 5bc2cdab..421a0cb1 100644
--- a/flopy4/mf6/codec/reader/grammar/typed.lark
+++ b/flopy4/mf6/codec/reader/grammar/typed.lark
@@ -58,14 +58,19 @@ open_close_redirect: "open/close"i filename [_remark] _NL
 %import common.WS_INLINE
 %import common.CNAME
 %import common.WORD
-%import common.NUMBER
-%import common.INT
 %import common.SH_COMMENT
 %import common.ESCAPED_STRING
-%import common.SIGNED_NUMBER
-%import common.SIGNED_INT
 
 COMMA: ","
 %ignore WS_INLINE
 %ignore SH_COMMENT
 %ignore COMMA
+
+// common's number terminals, but only as a whole token. Otherwise a token
+// that starts like a number lexes as a number followed by a word, e.g. an
+// ISO datetime `1997-07-16T19:20:30` as `1997`, `-07`, `-16` and
+// `T19:20:30`.
+INT: /\d+(?![^\s,#])/
+SIGNED_INT: /[+-]\d+(?![^\s,#])/
+NUMBER: /(\d+(\.\d*)?|\.\d+)([eE][+-]?\d+)?(?![^\s,#])/
+SIGNED_NUMBER: /[+-](\d+(\.\d*)?|\.\d+)([eE][+-]?\d+)?(?![^\s,#])/
diff --git a/flopy4/mf6/codec/reader/transformer/basic.py b/flopy4/mf6/codec/reader/transformer/basic.py
index f2468955..6fd596b3 100644
--- a/flopy4/mf6/codec/reader/transformer/basic.py
+++ b/flopy4/mf6/codec/reader/transformer/basic.py
@@ -1,9 +1,13 @@
+import re
 from typing import Any
 
 from lark import Token, Transformer
 
 from flopy4.utils import parse_number
 
+# A whole token that's a number, allowing Fortran `D` exponents (`1D-5`).
+_NUMBER = re.compile(r"[+-]?(\d+(\.\d*)?|\.\d+)([eEdD][+-]?\d+)?")
+
 
 class BasicTransformer(Transformer):
     """
@@ -41,14 +45,11 @@ def _list(self, items: list[Any]) -> list[Any]:
     def line(self, items: list[Any]) -> list[Any]:
         return items
 
-    def item(self, items: list[Any]) -> str | float | int:
-        return items[0]
-
-    def word(self, items: list[Token]) -> str:
-        return str(items[0])
-
-    def NUMBER(self, token: Token) -> int | float:
-        return parse_number(str(token))
+    def TOKEN(self, token: Token) -> str | int | float:
+        value = str(token)
+        if _NUMBER.fullmatch(value):
+            return parse_number(value.replace("d", "e").replace("D", "E"))
+        return value
 
     def CNAME(self, token: Token) -> str:
         return str(token)
diff --git a/test/mf6/test_mf6_codec.py b/test/mf6/test_mf6_codec.py
index ceae13fc..917125a9 100644
--- a/test/mf6/test_mf6_codec.py
+++ b/test/mf6/test_mf6_codec.py
@@ -38,6 +38,25 @@ def test_loads_dis_generic_simple():
     assert result["griddata"] == [["delr"], ["constant", 100.0], ["delc"], ["constant", 100.0]]
 
 
+def test_loads_number_only_as_whole_token():
+    mf6_input = """
+BEGIN options
+  start_date_time 1997-07-16T19:20:30.45+01:00
+  ts6 filein 1model.ts
+  x 1,2.5 .5 7. 1e5 3 # comment
+  y -5 +2 -1.5e3 1D-5 8.2d-4
+END options
+"""
+
+    result = loads(mf6_input)
+    assert result["options"] == [
+        ["start_date_time", "1997-07-16T19:20:30.45+01:00"],
+        ["ts6", "filein", "1model.ts"],
+        ["x", 1, 2.5, 0.5, 7.0, 1e5, 3],
+        ["y", -5, 2, -1500.0, 1e-5, 8.2e-4],
+    ]
+
+
 def test_dumps_ic():
     from flopy4.mf6.gwf import Dis, Gwf, Ic
 
diff --git a/test/mf6/test_mf6_io_roundtrip.py b/test/mf6/test_mf6_io_roundtrip.py
index cbefe711..630dd522 100644
--- a/test/mf6/test_mf6_io_roundtrip.py
+++ b/test/mf6/test_mf6_io_roundtrip.py
@@ -21,31 +21,6 @@
 from .test_mf6_load_all_models import KNOWN_PASSING
 
 XFAIL = {
-    # the basic grammar splits an ISO datetime (TDIS `START_DATE_TIME`) into
-    # a number and a word, so TDIS fails to write
-    "tokenizer splits ISO datetimes": {
-        "mf6/test/test001a_Tharmonic",
-        "mf6/test/test001a_Tharmonic_tabs",
-        "mf6/test/test001h_drn_list4",
-        "mf6/test/test001h_evt_array1",
-        "mf6/test/test001h_evt_array2",
-        "mf6/test/test001h_evt_array3",
-        "mf6/test/test001h_evt_array4",
-        "mf6/test/test001h_evt_list1",
-        "mf6/test/test001h_evt_list2",
-        "mf6/test/test001h_evt_list3",
-        "mf6/test/test001h_evt_list4",
-        "mf6/test/test001h_rch_array1",
-        "mf6/test/test001h_rch_array2",
-        "mf6/test/test001h_rch_array3",
-        "mf6/test/test001h_rch_array4",
-        "mf6/test/test001h_rch_list1",
-        "mf6/test/test001h_rch_list2",
-        "mf6/test/test001h_rch_list3",
-        "mf6/test/test001h_rch_list4",
-        "mf6/test/test001i_gwf-gwf",
-        "mf6/test/test001i_multilayer",
-    },
     # array values are written with 9 significant digits
     "writer rounds floats": {
         "mf6/test/test033_wtdecay",
@@ -86,6 +61,22 @@
     # `AUXILIARY <one name>` loads as a str, not a list, and egress drops it
     "ingress loads a single AUXILIARY name as a str": {
         "mf6/test/test001a_Tharmonic_extlist",
+        "mf6/test/test001h_evt_array1",
+        "mf6/test/test001h_evt_array2",
+        "mf6/test/test001h_evt_array3",
+        "mf6/test/test001h_evt_array4",
+        "mf6/test/test001h_evt_list1",
+        "mf6/test/test001h_evt_list2",
+        "mf6/test/test001h_evt_list3",
+        "mf6/test/test001h_evt_list4",
+        "mf6/test/test001h_rch_array1",
+        "mf6/test/test001h_rch_array2",
+        "mf6/test/test001h_rch_array3",
+        "mf6/test/test001h_rch_array4",
+        "mf6/test/test001h_rch_list1",
+        "mf6/test/test001h_rch_list2",
+        "mf6/test/test001h_rch_list3",
+        "mf6/test/test001h_rch_list4",
         "mf6/test/test005_advgw_tidal",
         "mf6/test/test201_gwtbuy-henryCHD",
         "mf6/test/test202_gwtbuy-henryCHDm",
@@ -96,6 +87,7 @@
     # a TIMEARRAYSERIES-sourced period array is dropped on load (see
     # test_tas_period_array_kept)
     "ingress drops TAS period arrays": {
+        "mf6/test/test001h_rch_array3",
         "mf6/test/test027_TimeseriesTest",
         "mf6/test/test027_TimeseriesTest_idomain",
     },
diff --git a/test/mf6/test_mf6_reader.py b/test/mf6/test_mf6_reader.py
index d44e7e34..163d0613 100644
--- a/test/mf6/test_mf6_reader.py
+++ b/test/mf6/test_mf6_reader.py
@@ -782,3 +782,23 @@ def test_typed_grammar_repeating_block_double_header(tmp_path):
     result = transformer.transform(parser.parse("BEGIN TIME 1.5\n    X CONSTANT 1.0\nEND TIME\n"))
 
     assert 1.5 in result["time"]
+
+
+def test_typed_loads_filename_with_leading_digit(dfn_path):
+    from flopy4.mf6.codec.reader import loads_typed
+
+    result = loads_typed(
+        "BEGIN options\n  ts6 filein 1model.ts\nEND options\n", "gwf-chd", dfn_path=dfn_path
+    )
+    assert result["options"]["ts_filerecord"]["ts6_filename"] == "1model.ts"
+
+
+def test_typed_loads_iso_datetime(dfn_path):
+    from flopy4.mf6.codec.reader import loads_typed
+
+    result = loads_typed(
+        "BEGIN options\n  start_date_time 1997-07-16T19:20:30.45+01:00\nEND options\n",
+        "sim-tdis",
+        dfn_path=dfn_path,
+    )
+    assert result["options"]["start_date_time"] == "1997-07-16T19:20:30.45+01:00"