diff --git a/imap/conversations.c b/imap/conversations.c
index 42618c809e..f94d22dca9 100644
--- a/imap/conversations.c
+++ b/imap/conversations.c
@@ -962,8 +962,8 @@ static void conv_to_buf(conversation_t *conv, struct buf *buf, int flagcount)
         nn = dlist_newlist(n, "THREAD");
         dlist_setguid(nn, "GUID", &thread->guid);
         dlist_setnum32(nn, "EXISTS", thread->exists);
-        dlist_setnum32(nn, "INTERNALDATE", thread->internaldate);
-        dlist_setnum32(nn, "CREATEDMODSEQ", thread->createdmodseq);
+        dlist_setnum64(nn, "INTERNALDATE", thread->internaldate);
+        dlist_setnum64(nn, "CREATEDMODSEQ", thread->createdmodseq);
     }
 
     dlist_setnum64(dl, "CREATEDMODSEQ", conv->createdmodseq);
@@ -1490,7 +1490,7 @@ int _saxconvparse(int type, struct dlistsax_data *d)
             return 0;
 
         case 2:
-            rock->thread->internaldate = atol(d->data);
+            rock->thread->internaldate = atoll(d->data);
             rock->substate = 3;
             return 0;
 
@@ -1775,8 +1775,12 @@ static int _thread_datesort(const void **a, const void **b)
 {
     const conv_thread_t *ta = (const conv_thread_t *)*a;
     const conv_thread_t *tb = (const conv_thread_t *)*b;
+    struct timespec ta_internaldate, tb_internaldate;
 
-    int r = (ta->internaldate - tb->internaldate);
+    TIMESPEC_FROM_NANOSEC(&ta_internaldate, ta->internaldate);
+    TIMESPEC_FROM_NANOSEC(&tb_internaldate, tb->internaldate);
+
+    int r = (ta_internaldate.tv_sec - tb_internaldate.tv_sec);
     if (r < 0) return -1;
     if (r > 0) return 1;
 
@@ -1817,7 +1821,7 @@ static void conversations_thread_sort(conversation_t *conv)
 
 static void conversation_update_thread(conversation_t *conv,
                                        const struct message_guid *guid,
-                                       time_t internaldate,
+                                       uint64_t internaldate,
                                        modseq_t createdmodseq,
                                        int delta_exists)
 {
@@ -1963,7 +1967,7 @@ static int _guid_one(struct guid_foreach_rock *frock,
                      conversation_id_t basecid,
                      uint32_t system_flags,
                      uint32_t internal_flags,
-                     time_t internaldate,
+                     uint64_t internaldate,
                      char version)
 {
     const char *p, *err;
@@ -2071,7 +2075,7 @@ static int _guid_cb(void *rock,
     conversation_id_t basecid = 0;
     uint32_t system_flags = 0;
     uint32_t internal_flags = 0;
-    time_t internaldate = 0;
+    uint64_t internaldate = 0;
     char version = 0;
     if (datalen >= 16) {
         const char *p = data;
@@ -2114,7 +2118,7 @@ static int _guid_cb(void *rock,
             internal_flags = ntohl(*((bit32*)p));
             p += 4;
             /* internaldate*/
-            internaldate = (time_t) ntohll(*((bit64*)p));
+            internaldate = ntohll(*((bit64*)p));
             p += 8;
             /* basecid */
             basecid = ntohll(*((bit64*)p));
@@ -2228,7 +2232,7 @@ static int conversations_guid_setitem(struct conversations_state *state,
                                       conversation_id_t basecid,
                                       uint32_t system_flags,
                                       uint32_t internal_flags,
-                                      time_t internaldate,
+                                      uint64_t internaldate,
                                       int add)
 {
     struct buf key = BUF_INITIALIZER;
@@ -2271,7 +2275,7 @@ static int conversations_guid_setitem(struct conversations_state *state,
             buf_appendbit64(&val, cid);
             buf_appendbit32(&val, system_flags);
             buf_appendbit32(&val, internal_flags);
-            buf_appendbit64(&val, (bit64)internaldate);
+            buf_appendbit64(&val, internaldate);
         }
         /* When bumping the G value version, make sure to update _guid_cb */
         else {
@@ -2279,7 +2283,7 @@ static int conversations_guid_setitem(struct conversations_state *state,
             buf_appendbit64(&val, cid);
             buf_appendbit32(&val, system_flags);
             buf_appendbit32(&val, internal_flags);
-            buf_appendbit64(&val, (bit64)internaldate);
+            buf_appendbit64(&val, internaldate);
             buf_appendbit64(&val, basecid == cid ? 0 : basecid);
         }
 
@@ -2302,7 +2306,7 @@ static int _guid_addbody(struct conversations_state *state,
                          conversation_id_t cid,
                          conversation_id_t basecid,
                          uint32_t system_flags, uint32_t internal_flags,
-                         time_t internaldate,
+                         uint64_t internaldate,
                          struct body *body,
                          const char *base, int add)
 {
@@ -2357,15 +2361,37 @@ static int conversations_set_guid(struct conversations_state *state,
     buf_printf(&item, "%d:%u", folder, record->uid);
     const char *base = buf_cstring(&item);
 
-    r = conversations_guid_setitem(state, message_guid_encode(&record->guid),
+    const char *guidrep = message_guid_encode(&record->guid);
+    uint64_t internaldate = TIMESPEC_TO_NANOSEC(&record->internaldate);
+    r = conversations_guid_setitem(state, guidrep,
                                    base, record->cid, record->basecid,
                                    record->system_flags,
                                    record->internal_flags,
-                                   record->internaldate.tv_sec,
+                                   internaldate,
                                    add);
+    if (!r) {
+        struct buf key = BUF_INITIALIZER;
+
+        /* Build J key */
+        buf_setcstr(&key, "J");
+        NANOSEC_TO_JMAPID(&key, internaldate);
+
+        if (add) {
+            /* Insert J record: jidrep -> guidrep */
+            r = cyrusdb_store(state->db, buf_base(&key), buf_len(&key),
+                              guidrep, strlen(guidrep),
+                              &state->txn);
+        }
+        else {
+            /* Remove J record */
+            r = cyrusdb_delete(state->db, buf_base(&key), buf_len(&key),
+                               &state->txn, /*force*/1);
+        }
+    }
     if (!r) r = _guid_addbody(state, record->cid, record->basecid,
                               record->system_flags, record->internal_flags,
-                              record->internaldate.tv_sec, body, base, add);
+                              internaldate,
+                              body, base, add);
 
     message_free_body(body);
     free(body);
@@ -2518,6 +2544,7 @@ EXPORTED int conversations_update_record(struct conversations_state *cstate,
     if (new) {
         if (!old || old->system_flags != new->system_flags ||
                     old->internal_flags != new->internal_flags ||
+                    old->internaldate.tv_nsec != new->internaldate.tv_nsec  ||
                     old->internaldate.tv_sec != new->internaldate.tv_sec) {
             r = conversations_set_guid(cstate, mailbox, new, /*add*/1);
             if (r) goto done;
@@ -2634,7 +2661,7 @@ EXPORTED int conversations_update_record(struct conversations_state *cstate,
 
     conversation_update_thread(conv,
                                &record->guid,
-                               record->internaldate.tv_sec,
+                               TIMESPEC_TO_NANOSEC(&record->internaldate),
                                record->createdmodseq,
                                delta_exists);
 
diff --git a/imap/conversations.h b/imap/conversations.h
index f8224a317b..addfd73e62 100644
--- a/imap/conversations.h
+++ b/imap/conversations.h
@@ -124,7 +124,7 @@ struct conv_thread {
     conv_thread_t *next;
     struct message_guid guid;
     uint32_t exists;
-    time_t internaldate;
+    uint64_t internaldate; // nanoseconds since epoch
     modseq_t createdmodseq;
 };
 
@@ -138,7 +138,7 @@ struct conv_folder {
     uint32_t        prev_exists;
 };
 
-#define CONV_GUIDREC_VERSION 3          // (must be <= 127)
+#define CONV_GUIDREC_VERSION 4          // (must be <= 127)
 #define CONV_GUIDREC_BYNAME_VERSION 1   // last folders byname version
 
 struct conv_guidrec {
@@ -148,11 +148,11 @@ struct conv_guidrec {
     uint32_t        uid;
     const char      *part;
     conversation_id_t cid;
-    conversation_id_t basecid;
+    conversation_id_t basecid;      // if version >= 3
     char            version;
     uint32_t        system_flags;   // if version >= 1
     uint32_t        internal_flags; // if version >= 1
-    time_t          internaldate;   // if version >= 1
+    uint64_t        internaldate;   // if version >= 4 (nanoseconds since epoch)
 };
 
 struct conv_sender {
diff --git a/imap/jmap_mail.c b/imap/jmap_mail.c
index fce3a5f2b8..5befba23b1 100644
--- a/imap/jmap_mail.c
+++ b/imap/jmap_mail.c
@@ -3235,9 +3235,9 @@ static int emailsearch_is_mutable(struct emailsearch *search)
 struct guidsearch_match {
     char guidrep[MESSAGE_GUID_SIZE*2+1];
     uint32_t system_flags;
-    uint32_t internaldate;
+    uint64_t internaldate;  // nanoseconds since epoch
     conversation_id_t cid;
-    bitvector_t folders; // only set if numfolders > 0
+    bitvector_t folders;    // only set if numfolders > 0
 };
 
 static void guidsearch_match_init(struct guidsearch_match *match,
@@ -3261,10 +3261,14 @@ static int guidsearch_match_cmp QSORT_R_COMPAR_ARGS(const void *va,
     while (sort->key != SORT_SEQUENCE) {
         int ret;
         switch (sort->key) {
-            case SORT_ARRIVAL:
-                ret = a->internaldate < b->internaldate ? -1 :
-                      a->internaldate > b->internaldate ?  1 : 0;
+            case SORT_ARRIVAL: {
+                struct timespec a_internaldate, b_internaldate;
+                TIMESPEC_FROM_NANOSEC(&a_internaldate, a->internaldate);
+                TIMESPEC_FROM_NANOSEC(&b_internaldate, b->internaldate);
+                ret = a_internaldate.tv_sec < b_internaldate.tv_sec ? -1 :
+                      a_internaldate.tv_sec > b_internaldate.tv_sec ?  1 : 0;
                 break;
+            }
             case SORT_GUID:
                 ret = memcmp(a->guidrep, b->guidrep, MESSAGE_GUID_SIZE*2);
                 break;
@@ -13223,7 +13227,7 @@ static void _email_bulkupdate_exec_setflags(struct email_bulkupdate *bulk)
 
             if (update->received_at) {
                 /* Write internaldate (Email/copy only) */
-              struct timespec internaldate = { 0 };
+                struct timespec internaldate = { 0 };
                 time_from_iso8601(update->received_at, &internaldate.tv_sec);
                 r = msgrecord_set_internaldate(mrw, &internaldate);
             }
diff --git a/lib/util.h b/lib/util.h
index 516f9d30fc..7aa82d9d23 100644
--- a/lib/util.h
+++ b/lib/util.h
@@ -162,9 +162,14 @@ extern const unsigned char convert_to_uppercase[256];
 
 #define TIMESPEC_TO_NANOSEC(ts) ((ts)->tv_sec * 1000000000 + (ts)->tv_nsec)
 
-#define TIMESPEC_FROM_NANOSEC(ts, nanosec) {  \
-        (ts)->tv_sec  = nanosec / 1000000000; \
-        (ts)->tv_nsec = nanosec % 1000000000; \
+#define TIMESPEC_FROM_NANOSEC(ts, nanosec) {    \
+        (ts)->tv_sec  = (nanosec) / 1000000000; \
+        (ts)->tv_nsec = (nanosec) % 1000000000; \
+}
+
+#define NANOSEC_TO_JMAPID(buf, nanosec) {                                   \
+        uint64_t u64 = htonll(UINT64_MAX - (nanosec));                      \
+        charset_encode(buf, (const char *) &u64, 8, ENCODING_BASE64JMAPID); \
 }
 
 typedef struct keyvalue {