Index: src/date.c
==================================================================
--- src/date.c
+++ src/date.c
@@ -75,10 +75,11 @@
   char validYMD;      /* True (1) if Y,M,D are valid */
   char validHMS;      /* True (1) if h,m,s are valid */
   char validTZ;       /* True (1) if tz is valid */
   char tzSet;         /* Timezone was set explicitly */
   char isError;       /* An overflow has occurred */
+  char useSubsec;     /* Display subsecond precision */
 };
 
 
 /*
 ** Convert zDate into one or more integers according to the conversion
@@ -389,10 +390,15 @@
   }else if( sqlite3StrICmp(zDate,"now")==0 && sqlite3NotPureFunc(context) ){
     return setDateTimeToCurrent(context, p);
   }else if( sqlite3AtoF(zDate, &r, sqlite3Strlen30(zDate), SQLITE_UTF8)>0 ){
     setRawDateNumber(p, r);
     return 0;
+  }else if( (sqlite3StrICmp(zDate,"subsec")==0
+             || sqlite3StrICmp(zDate,"subsecond")==0)
+           && sqlite3NotPureFunc(context) ){
+    p->useSubsec = 1;
+    return setDateTimeToCurrent(context, p);
   }
   return 1;
 }
 
 /* The julian day number for 9999-12-31 23:59:59.999 is 5373484.4999999.
@@ -803,12 +809,26 @@
       /*
       **    start of TTTTT
       **
       ** Move the date backwards to the beginning of the current day,
       ** or month or year.
+      **
+      **    subsecond
+      **    subsec
+      **
+      ** Show subsecond precision in the output of datetime() and
+      ** unixepoch() and strftime('%s').
       */
-      if( sqlite3_strnicmp(z, "start of ", 9)!=0 ) break;
+      if( sqlite3_strnicmp(z, "start of ", 9)!=0 ){
+        if( sqlite3_stricmp(z, "subsec")==0
+         || sqlite3_stricmp(z, "subsecond")==0
+        ){
+          p->useSubsec = 1;
+          rc = 0;
+        }
+        break;
+      }        
       if( !p->validJD && !p->validYMD && !p->validHMS ) break;
       z += 9;
       computeYMD(p);
       p->validHMS = 1;
       p->h = p->m = 0;
@@ -1002,11 +1022,15 @@
   sqlite3_value **argv
 ){
   DateTime x;
   if( isDate(context, argc, argv, &x)==0 ){
     computeJD(&x);
-    sqlite3_result_int64(context, x.iJD/1000 - 21086676*(i64)10000);
+    if( x.useSubsec ){
+      sqlite3_result_double(context, (x.iJD - 21086676*(i64)10000000)/1000.0);
+    }else{
+      sqlite3_result_int64(context, x.iJD/1000 - 21086676*(i64)10000);
+    }
   }
 }
 
 /*
 **    datetime( TIMESTRING, MOD, MOD, ...)
@@ -1018,12 +1042,12 @@
   int argc,
   sqlite3_value **argv
 ){
   DateTime x;
   if( isDate(context, argc, argv, &x)==0 ){
-    int Y, s;
-    char zBuf[24];
+    int Y, s, n;
+    char zBuf[32];
     computeYMD_HMS(&x);
     Y = x.Y;
     if( Y<0 ) Y = -Y;
     zBuf[1] = '0' + (Y/1000)%10;
     zBuf[2] = '0' + (Y/100)%10;
@@ -1040,19 +1064,32 @@
     zBuf[13] = '0' + (x.h)%10;
     zBuf[14] = ':';
     zBuf[15] = '0' + (x.m/10)%10;
     zBuf[16] = '0' + (x.m)%10;
     zBuf[17] = ':';
-    s = (int)x.s;
-    zBuf[18] = '0' + (s/10)%10;
-    zBuf[19] = '0' + (s)%10;
-    zBuf[20] = 0;
+    if( x.useSubsec ){
+      s = (int)1000.0*x.s;
+      zBuf[18] = '0' + (s/10000)%10;
+      zBuf[19] = '0' + (s/1000)%10;
+      zBuf[20] = '.';
+      zBuf[21] = '0' + (s/100)%10;
+      zBuf[22] = '0' + (s/10)%10;
+      zBuf[23] = '0' + (s)%10;
+      zBuf[24] = 0;
+      n = 24;
+    }else{
+      s = (int)x.s;
+      zBuf[18] = '0' + (s/10)%10;
+      zBuf[19] = '0' + (s)%10;
+      zBuf[20] = 0;
+      n = 20;
+    }
     if( x.Y<0 ){
       zBuf[0] = '-';
-      sqlite3_result_text(context, zBuf, 20, SQLITE_TRANSIENT);
+      sqlite3_result_text(context, zBuf, n, SQLITE_TRANSIENT);
     }else{
-      sqlite3_result_text(context, &zBuf[1], 19, SQLITE_TRANSIENT);
+      sqlite3_result_text(context, &zBuf[1], n-1, SQLITE_TRANSIENT);
     }
   }
 }
 
 /*
@@ -1065,24 +1102,37 @@
   int argc,
   sqlite3_value **argv
 ){
   DateTime x;
   if( isDate(context, argc, argv, &x)==0 ){
-    int s;
+    int s, n;
     char zBuf[16];
     computeHMS(&x);
     zBuf[0] = '0' + (x.h/10)%10;
     zBuf[1] = '0' + (x.h)%10;
     zBuf[2] = ':';
     zBuf[3] = '0' + (x.m/10)%10;
     zBuf[4] = '0' + (x.m)%10;
     zBuf[5] = ':';
-    s = (int)x.s;
-    zBuf[6] = '0' + (s/10)%10;
-    zBuf[7] = '0' + (s)%10;
-    zBuf[8] = 0;
-    sqlite3_result_text(context, zBuf, 8, SQLITE_TRANSIENT);
+    if( x.useSubsec ){
+      s = (int)1000.0*x.s;
+      zBuf[6] = '0' + (s/10000)%10;
+      zBuf[7] = '0' + (s/1000)%10;
+      zBuf[8] = '.';
+      zBuf[9] = '0' + (s/100)%10;
+      zBuf[10] = '0' + (s/10)%10;
+      zBuf[11] = '0' + (s)%10;
+      zBuf[12] = 0;
+      n = 12;
+    }else{
+      s = (int)x.s;
+      zBuf[6] = '0' + (s/10)%10;
+      zBuf[7] = '0' + (s)%10;
+      zBuf[8] = 0;
+      n = 8;
+    }
+    sqlite3_result_text(context, zBuf, n, SQLITE_TRANSIENT);
   }
 }
 
 /*
 **    date( TIMESTRING, MOD, MOD, ...)
@@ -1209,12 +1259,17 @@
       case 'M': {
         sqlite3_str_appendf(&sRes,"%02d",x.m);
         break;
       }
       case 's': {
-        i64 iS = (i64)(x.iJD/1000 - 21086676*(i64)10000);
-        sqlite3_str_appendf(&sRes,"%lld",iS);
+        if( x.useSubsec ){
+          sqlite3_str_appendf(&sRes,"%.3f",
+                (x.iJD - 21086676*(i64)10000000)/1000.0);
+        }else{
+          i64 iS = (i64)(x.iJD/1000 - 21086676*(i64)10000);
+          sqlite3_str_appendf(&sRes,"%lld",iS);
+        }
         break;
       }
       case 'S': {
         sqlite3_str_appendf(&sRes,"%02d",(int)x.s);
         break;

Index: test/date.test
==================================================================
--- test/date.test
+++ test/date.test
@@ -542,6 +542,12 @@
 # 2022-03-04 https://sqlite.org/forum/forumpost/2ffbaa2c3fd7fb82
 # The 'localtime' modifier should preserve fractional seconds.
 #
 datetest 18.1 {strftime('%f',1.234,'unixepoch','localtime')} {01.234}
 
+# 2023-04 The 'subsecond' (or 'subsec') modifier alters resolutions
+# to at least milliseconds. Added for release 3.42.0 .
+datetest 18.2 {unixepoch('1970-01-01T00:00:00.1', 'subsec')} {0.1}
+datetest 18.3 {unixepoch('1970-01-01T00:00:00.2', 'subsecond')} {0.2}
+datetest 18.4 {julianday('-4713-11-24 13:40:48.864', 'subsec')} {0.07001}
+datetest 18.5 {typeof(unixepoch('now', 'subsecond'))} {real}
 finish_test