diff --git a/cmd/cullbak/README.md b/cmd/cullbak/README.md
new file mode 100644
index 0000000..c6ccde7
--- /dev/null
+++ b/cmd/cullbak/README.md
@@ -0,0 +1,41 @@
+# Cull Backups
+
+Culls backups on a monthly, weekly, and daily schedule
+
+Keeps 1 backup for
+
+- each iso week, forever
+- each day for the last 2 weeks
+- each backup, for the last 7 days
+
+The backups are done on whatever schedule by some other process.
+
+This process is called on the backups folder, after each backup completes.
+
+## Usage
+
+```sh
+for my_dir in $(ls -d /mnt/backups/*); do
+    cull-backups --dry-run --keep-dailies 35 --exts 'tar.zst,sql.zst,tar.xz,sql.xz,' "/mnt/backups/${my_dir}"
+done
+```
+
+```text
+/mnt/
+└── backups/
+    ├── proj-y/
+    │   ├── monthly/
+    │   ├── weekly/
+    │   ├── daily/
+    │   └── 2024-01-01_00.00.00.tar.xz
+    ├── project-x/
+    │   ├── monthly/
+    │   ├── weekly/
+    │   ├── daily/
+    │   └── 2024-01-01_00.00.00.tar.xz
+    └── projectfoo-z/
+        ├── monthly/
+        ├── weekly/
+        ├── daily/
+        └── 2024-01-01_00.00.00.tar.xz
+```
diff --git a/cmd/cullbak/cullbak.go b/cmd/cullbak/cullbak.go
new file mode 100644
index 0000000..2c2187e
--- /dev/null
+++ b/cmd/cullbak/cullbak.go
@@ -0,0 +1,239 @@
+// Authored in 2024 by AJ ONeal <aj@therootcompany.com> (https://therootcompany.com)
+//
+// To the extent possible under law, the author(s) have dedicated all copyright
+// and related and neighboring rights to this software to the public domain
+// worldwide. This software is distributed without any warranty.
+//
+// You should have received a copy of the CC0 Public Domain Dedication along with
+// this software. If not, see <https://creativecommons.org/publicdomain/zero/1.0/>.
+//
+// SPDX-License-Identifier: CC0-1.0
+
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+	"time"
+)
+
+const (
+	dailyDir  = "."
+	weeklyDir = "weekly"
+	trashDir  = "trash"
+)
+
+var (
+	dailyExpire  time.Duration
+	exts         []string
+	visitedWeeks = make(map[string]struct{})
+)
+
+func main() {
+	var path string
+	var extsFlag string
+	var dryRun bool
+	var keepDaily int
+
+	flag.StringVar(&extsFlag, "exts", "", "comma-separated list of file extensions")
+	flag.IntVar(&keepDaily, "keep-daily", 5*7, "how many daily backups to retain")
+	flag.BoolVar(&dryRun, "dry-run", false, "print files that would be moved or deleted")
+	flag.Parse()
+
+	if flag.NArg() != 1 {
+		fmt.Println("USAGE")
+		fmt.Println("    go run cullbak.go --exts 'tar.xz,tar.zst' /mnt/backups/project-x/")
+		os.Exit(1)
+	}
+	path = flag.Arg(0)
+
+	dailyExpire = time.Duration(keepDaily) * 24 * time.Hour
+
+	for ext := range strings.SplitSeq(extsFlag, ",") {
+		ext = strings.TrimSpace(ext)
+		if ext == "" {
+			continue
+		}
+		if !strings.HasPrefix(ext, ".") {
+			ext = "." + ext
+		}
+		exts = append(exts, ext)
+	}
+	if len(exts) == 0 {
+		fmt.Fprintln(os.Stderr, "no valid extensions provided")
+		os.Exit(1)
+	}
+
+	// Get the current date
+	currentDay := time.Now()
+	fmt.Println("Culling archive files of these types:", strings.Join(exts, ","))
+
+	// Process daily backups
+	err := processDailyBackups(path, currentDay, dryRun)
+	if err != nil {
+		fmt.Printf("Error processing %q: %v\n", path, err)
+	}
+}
+
+func processDailyBackups(
+	path string, currentDay time.Time, dryRun bool,
+) error {
+	//bakfiles := []os.DirEntry{}
+	bakfiles := []os.FileInfo{}
+	dailyPath := filepath.Join(path, dailyDir)
+
+	{
+		allFiles, err := os.ReadDir(dailyPath)
+		if err != nil {
+			return err
+		}
+
+		for _, file := range allFiles {
+			if file.IsDir() {
+				fmt.Printf("[skip] directory %q\n", file.Name())
+				continue
+			}
+
+			hasExt := false
+			for _, ext := range exts {
+				if strings.HasSuffix(file.Name(), ext) {
+					hasExt = true
+					break
+				}
+			}
+			if !hasExt {
+				fmt.Printf("[skip] non-archive file %q\n", file.Name())
+				continue
+			}
+
+			fileInfo, err := file.Info()
+			if err != nil {
+				return err
+			}
+			bakfiles = append(bakfiles, fileInfo)
+		}
+	}
+
+	// oldest to youngest (asc)
+	sort.Slice(bakfiles, func(i, j int) bool {
+		fileI := bakfiles[i]
+		fileJ := bakfiles[j]
+
+		return fileI.ModTime().Before(fileJ.ModTime())
+	})
+
+	for _, fileInfo := range bakfiles {
+		modifiedTime := fileInfo.ModTime()
+		age := currentDay.Sub(modifiedTime)
+		if dailyExpire < 24*time.Hour {
+			panic("'keep-daily' must be at least 1 day")
+		}
+		tooFreshToCull := age < dailyExpire
+		if tooFreshToCull {
+			fmt.Printf("[skip] files from this and after are fresh: %q (%s)\n", fileInfo.Name(), formatDuration(age))
+			break
+		}
+
+		year, week := modifiedTime.ISOWeek()
+		yearWeek := fmt.Sprintf("%d-w%02d", year, week)
+		weeklyPath := filepath.Join(path, weeklyDir, yearWeek)
+
+		// If the dir exists, skip (we already have that backup)
+		if _, ok := visitedWeeks[weeklyPath]; !ok {
+			if _, err := os.Stat(weeklyPath); os.IsNotExist(err) {
+				src := filepath.Join(dailyPath, fileInfo.Name())
+				dst := filepath.Join(weeklyPath, fileInfo.Name())
+
+				if dryRun {
+					fmt.Printf("mv %q %q\n", src, dst)
+					visitedWeeks[weeklyPath] = struct{}{}
+					continue
+				}
+
+				if err := os.MkdirAll(weeklyPath, os.ModePerm); err != nil {
+					return err
+				}
+				err := os.Rename(src, dst)
+				if err != nil {
+					fmt.Printf("couldn't move %s to %s:\n    %v\n", src, dst, err)
+					// remove the dir so that it doesn't count as existing
+					_ = os.Remove(weeklyPath)
+					return err
+				}
+				fmt.Println("[move] created weekly backup", src, weeklyPath)
+				continue
+			}
+		}
+
+		{
+			trashPath := filepath.Join(path, trashDir)
+			src := filepath.Join(dailyPath, fileInfo.Name())
+			dst := filepath.Join(trashPath, fileInfo.Name())
+
+			if dryRun {
+				fmt.Printf("rm %s\n", src)
+				continue
+			}
+
+			if err := os.MkdirAll(trashPath, os.ModePerm); err != nil {
+				return err
+			}
+
+			fmt.Printf("[cull] delete %s\n", src)
+			if err := os.Rename(src, dst); err != nil {
+				fmt.Printf("couldn't move %s to %s:\n    %v\n", src, dst, err)
+				return err
+			}
+			if err := os.Remove(dst); err != nil {
+				fmt.Printf("couldn't delete %s: %v\n", dst, err)
+			}
+		}
+	}
+
+	return nil
+}
+
+func formatDuration(d time.Duration) string {
+	days := int64(d / (24 * time.Hour))
+	d -= time.Duration(days) * 24 * time.Hour
+	hours := int64(d / time.Hour)
+	d -= time.Duration(hours) * time.Hour
+	minutes := int64(d / time.Minute)
+	d -= time.Duration(minutes) * time.Minute
+	seconds := d.Seconds()
+
+	var parts []string
+	if days > 0 {
+		parts = append(parts, fmt.Sprintf("%dd", days))
+	}
+	if hours > 0 || days > 0 {
+		parts = append(parts, fmt.Sprintf("%dh", hours))
+	}
+	if minutes > 0 || hours > 0 || days > 0 {
+		parts = append(parts, fmt.Sprintf("%dm", minutes))
+	}
+
+	if hours < 1 {
+		parts = append(parts, fmt.Sprintf("%.3fs", seconds))
+	} else {
+		parts = append(parts, fmt.Sprintf("%ds", int64(seconds)))
+	}
+
+	return strings.Join(parts, " ")
+}
+
+// func firstDayOfWeek90DaysAgo(currentDay time.Time) time.Time {
+// 	// Calculate 90 days ago
+// 	daysAgo := currentDay.AddDate(0, 0, -90)
+
+// 	// Calculate the first day (Sunday) of the week 90 days ago
+// 	daysAgoWeekday := daysAgo.Weekday()
+// 	daysToFirstDayOfWeek := int(time.Sunday - daysAgoWeekday)
+// 	firstDayOfWeek := daysAgo.AddDate(0, 0, daysToFirstDayOfWeek)
+
+// 	return firstDayOfWeek
+// }