diff --git a/go/cli/mcap/cmd/attachment.go b/go/cli/mcap/cmd/attachment.go
index ee44cdf2e..2f868f06e 100644
--- a/go/cli/mcap/cmd/attachment.go
+++ b/go/cli/mcap/cmd/attachment.go
@@ -13,9 +13,9 @@ import (
 )
 
 var (
-	addAttachmentLogTime      uint64
+	addAttachmentLogTime      string
 	addAttachmentName         string
-	addAttachmentCreationTime uint64
+	addAttachmentCreationTime string
 	addAttachmentFilename     string
 	addAttachmentMediaType    string
 )
@@ -155,12 +155,20 @@ var addAttachmentCmd = &cobra.Command{
 			die("failed to stat file %s", addAttachmentFilename)
 		}
 		createTime := uint64(fi.ModTime().UTC().UnixNano())
-		if addAttachmentCreationTime > 0 {
-			createTime = addAttachmentCreationTime
+		if addAttachmentCreationTime != "" {
+			date, err := parseDateOrNanos(addAttachmentCreationTime)
+			if err != nil {
+				die("failed to parse creation date: %s", err)
+			}
+			createTime = date
 		}
 		logTime := uint64(time.Now().UTC().UnixNano())
-		if addAttachmentLogTime > 0 {
-			logTime = addAttachmentLogTime
+		if addAttachmentLogTime != "" {
+			date, err := parseDateOrNanos(addAttachmentCreationTime)
+			if err != nil {
+				die("failed to parse log date: %s", err)
+			}
+			logTime = date
 		}
 		err = utils.AmendMCAP(f, []*mcap.Attachment{
 			{
@@ -187,11 +195,19 @@ func init() {
 	addAttachmentCmd.PersistentFlags().StringVarP(
 		&addAttachmentMediaType, "content-type", "", "application/octet-stream", "content type of attachment",
 	)
-	addAttachmentCmd.PersistentFlags().Uint64VarP(
-		&addAttachmentLogTime, "log-time", "", 0, "attachment log time in nanoseconds (defaults to current timestamp)",
+	addAttachmentCmd.PersistentFlags().StringVarP(
+		&addAttachmentLogTime,
+		"log-time",
+		"",
+		"",
+		"attachment log time in nanoseconds or RFC3339 format (defaults to current timestamp)",
 	)
-	addAttachmentCmd.PersistentFlags().Uint64VarP(
-		&addAttachmentLogTime, "creation-time", "", 0, "attachment creation time in nanoseconds (defaults to ctime)",
+	addAttachmentCmd.PersistentFlags().StringVarP(
+		&addAttachmentLogTime,
+		"creation-time",
+		"",
+		"",
+		"attachment creation time in nanoseconds or RFC3339 format (defaults to ctime)",
 	)
 	err := addAttachmentCmd.MarkPersistentFlagRequired("file")
 	if err != nil {
diff --git a/go/cli/mcap/cmd/filter.go b/go/cli/mcap/cmd/filter.go
index c92c9cc6a..e7a5704d1 100644
--- a/go/cli/mcap/cmd/filter.go
+++ b/go/cli/mcap/cmd/filter.go
@@ -8,6 +8,8 @@ import (
 	"math"
 	"os"
 	"regexp"
+	"strconv"
+	"time"
 
 	"github.com/foxglove/mcap/go/cli/mcap/utils"
 	"github.com/foxglove/mcap/go/mcap"
@@ -22,6 +24,8 @@ type filterFlags struct {
 	endSec             uint64
 	startNano          uint64
 	endNano            uint64
+	start              string
+	end                string
 	includeMetadata    bool
 	includeAttachments bool
 	outputCompression  string
@@ -41,20 +45,51 @@ type filterOpts struct {
 	chunkSize          int64
 }
 
+// parseDateOrNanos parses a string containing either an RFC3339-formatted date with timezone
+// or a decimal number of nanoseconds. It returns a uint64 timestamp in nanoseconds.
+func parseDateOrNanos(dateOrNanos string) (uint64, error) {
+	intNanos, err := strconv.ParseUint(dateOrNanos, 10, 64)
+	if err == nil {
+		return intNanos, nil
+	}
+	date, err := time.Parse(time.RFC3339, dateOrNanos)
+	if err != nil {
+		return 0, err
+	}
+	return uint64(date.UnixNano()), nil
+}
+
+// parseTimestampArgs implements the semantics for setting start and end times in the CLI.
+// a non-default value in `dateOrNanos` overrides `nanoseconds`, which overrides `seconds`.
+func parseTimestampArgs(dateOrNanos string, nanoseconds uint64, seconds uint64) (uint64, error) {
+	if dateOrNanos != "" {
+		return parseDateOrNanos(dateOrNanos)
+	}
+	if nanoseconds != 0 {
+		return nanoseconds, nil
+	}
+	return seconds * 1e9, nil
+}
+
 func buildFilterOptions(flags *filterFlags) (*filterOpts, error) {
 	opts := &filterOpts{
 		output:             flags.output,
 		includeMetadata:    flags.includeMetadata,
 		includeAttachments: flags.includeAttachments,
 	}
-	opts.start = flags.startNano
 	if flags.startSec > 0 {
 		opts.start = flags.startSec * 1e9
 	}
-	opts.end = flags.endNano
-	if flags.endSec > 0 {
-		opts.end = flags.endSec * 1e9
+	start, err := parseTimestampArgs(flags.start, flags.startNano, flags.startSec)
+	if err != nil {
+		return nil, fmt.Errorf("invalid start: %w", err)
 	}
+	opts.start = start
+	end, err := parseTimestampArgs(flags.end, flags.endSec, flags.endNano)
+	if err != nil {
+		return nil, fmt.Errorf("invalid end: %w", err)
+	}
+	opts.end = end
 	if opts.end == 0 {
 		opts.end = math.MaxUint64
 	}
@@ -392,30 +427,43 @@ usage:
 			[]string{},
 			"messages with topic names matching this regex will be excluded, can be supplied multiple times",
 		)
+		start := filterCmd.PersistentFlags().StringP(
+			"start",
+			"S",
+			"",
+			"only include messages logged at or after this time. Accepts integer nanoseconds or RFC3339-formatted date.",
+		)
 		startSec := filterCmd.PersistentFlags().Uint64P(
 			"start-secs",
 			"s",
 			0,
-			"messages with log times after or equal to this timestamp will be included.",
+			"only include messages logged at or after this time. Accepts integer seconds."+
+				"Ignored if `--start` or `--start-nsecs` are used.",
+		)
+		startNano := filterCmd.PersistentFlags().Uint64(
+			"start-nsecs",
+			0,
+			"deprecated, use --start. Only include messages logged at or after this time. Accepts integer nanoseconds.",
+		)
+		end := filterCmd.PersistentFlags().StringP(
+			"end",
+			"E",
+			"",
+			"Only include messages logged before this time. Accepts integer nanoseconds or RFC3339-formatted date.",
 		)
 		endSec := filterCmd.PersistentFlags().Uint64P(
 			"end-secs",
 			"e",
 			0,
-			"messages with log times before timestamp will be included.",
+			"only include messages logged before this time. Accepts integer seconds."+
+				"Ignored if `--end` or `--end-nsecs` are used.",
 		)
-		startNano := filterCmd.PersistentFlags().Uint64P(
-			"start-nsecs",
-			"S",
-			0,
-			"messages with log times after or equal to this nanosecond-precision timestamp will be included.",
-		)
-		endNano := filterCmd.PersistentFlags().Uint64P(
+		endNano := filterCmd.PersistentFlags().Uint64(
 			"end-nsecs",
-			"E",
 			0,
-			"messages with log times before nanosecond-precision timestamp will be included.",
+			"(Deprecated, use --end) Only include messages logged before this time. Accepts integer nanosconds.",
 		)
+
 		filterCmd.MarkFlagsMutuallyExclusive("start-secs", "start-nsecs")
 		filterCmd.MarkFlagsMutuallyExclusive("end-secs", "end-nsecs")
 		chunkSize := filterCmd.PersistentFlags().Int64P("chunk-size", "", 4*1024*1024, "chunk size of output file")
@@ -439,9 +487,11 @@ usage:
 				output:             *output,
 				includeTopics:      *includeTopics,
 				excludeTopics:      *excludeTopics,
+				start:              *start,
 				startSec:           *startSec,
-				endSec:             *endSec,
 				startNano:          *startNano,
+				end:                *end,
+				endSec:             *endSec,
 				endNano:            *endNano,
 				chunkSize:          *chunkSize,
 				includeMetadata:    *includeMetadata,
diff --git a/go/cli/mcap/cmd/filter_test.go b/go/cli/mcap/cmd/filter_test.go
index 32d6f9824..236dd8ca1 100644
--- a/go/cli/mcap/cmd/filter_test.go
+++ b/go/cli/mcap/cmd/filter_test.go
@@ -368,3 +368,13 @@ func TestCompileMatchers(t *testing.T) {
 	assert.True(t, matchers[0].MatchString("camera"))
 	assert.True(t, matchers[1].MatchString("lights"))
 }
+
+func TestParseDateOrNanos(t *testing.T) {
+	expected := uint64(1690298850132545471)
+	zulu, err := parseDateOrNanos("2023-07-25T15:27:30.132545471Z")
+	require.NoError(t, err)
+	assert.Equal(t, expected, zulu)
+	withTimezone, err := parseDateOrNanos("2023-07-26T01:27:30.132545471+10:00")
+	require.NoError(t, err)
+	assert.Equal(t, expected, withTimezone)
+}