diff --git a/README.md b/README.md
index dd7228d..e77b2f0 100644
--- a/README.md
+++ b/README.md
@@ -113,9 +113,10 @@ To build the base vm image:
 
 - Download images into /var/lib/libvirt/images
     - Windows 10 (multi-edition ISO), English (United States): [Win10_22H2_English_x64v1.iso](https://www.microsoft.com/en-us/software-download/windows10ISO) (sha256 = a6f470ca6d331eb353b815c043e327a347f594f37ff525f17764738fe812852e)
-- Run the build script: `windows10/build-image.sh`
+- Run the build script: `windows10/build-image.sh <pick a snapshot name>`
     - FIXME: if Windows fails to autologon (stuck at lock screen): `virsh reboot servo-windows10.new`
-- Update the profile config in `monitor/monitor.toml` to point to the new zvol snapshot, e.g. `base_image_snapshot = "build-image-2025-01-01T11:30Z"`
+- Update the profile config in `monitor/monitor.toml` to point to the new zvol snapshot, e.g. `base_image_snapshot = "<snapshot name>"`
+- Restart the monitor: `systemctl restart monitor`
 
 To clone and start a new runner:
 
@@ -130,8 +131,9 @@ Runners created from this image preinstall all dependencies (including those spe
 
 To build the base vm image:
 
-- Run the build script: `ubuntu2204/build-image.sh`
-- Update the profile config in `monitor/monitor.toml` to point to the new zvol snapshot, e.g. `base_image_snapshot = "build-image-2025-01-01T11:30Z"`
+- Run the build script: `ubuntu2204/build-image.sh <pick a snapshot name>`
+- Update the profile config in `monitor/monitor.toml` to point to the new zvol snapshot, e.g. `base_image_snapshot = "<snapshot name>"`
+- Restart the monitor: `systemctl restart monitor`
 
 To clone and start a new runner:
 
diff --git a/ubuntu2204/build-image.sh b/ubuntu2204/build-image.sh
index 731f656..db3b089 100755
--- a/ubuntu2204/build-image.sh
+++ b/ubuntu2204/build-image.sh
@@ -1,5 +1,5 @@
 #!/usr/bin/env zsh
-# usage: ubuntu2204/build-image.sh
+# usage: ubuntu2204/build-image.sh <snapshot_name>
 image_dir=${0:a:h}
 script_dir=${0:a:h}/..
 . "$script_dir/common.sh"
@@ -9,6 +9,7 @@ cache_dir=$script_dir/cache
 . "$script_dir/inject.sh"
 undo_commands=$(mktemp)
 image_name=servo-ubuntu2204
+snapshot_name=$1; shift
 
 >&2 echo '[*] Caching downloads'
 download "$cache_dir" https://cloud-images.ubuntu.com/jammy/20241217/jammy-server-cloudimg-amd64.img 0d8345a343c2547e55ac815342e6cb4a593aa5556872651eb47e6856a2bb0cdd
@@ -21,6 +22,7 @@ zfs list -Ho name "$SERVO_CI_ZFS_CLONE_PREFIX/$image_name" || zfs create -V 90G
 
 >&2 echo '[*] Creating libvirt guest (or recreating it with new config)'
 if virsh domstate -- "$image_name"; then
+    virsh destroy -- "$image_name" || :  # FIXME make this idempotent in a less noisy way?
     virsh undefine -- "$image_name"
 fi
 virsh define -- "$image_dir/guest.xml"
@@ -74,8 +76,7 @@ fi
 >&2 echo '[*] Checking that Servo was built correctly'
 ./mount-runner.sh "$image_name" sh -c 'ls init/built_servo_once_successfully'
 
-snapshot=$(date -u +\%FT\%RZ)
->&2 echo "[*] Taking zvol snapshot: $SERVO_CI_ZFS_CLONE_PREFIX/$image_name@build-image-$snapshot"
-zfs snapshot "$SERVO_CI_ZFS_CLONE_PREFIX/$image_name@build-image-$snapshot"
+>&2 echo "[*] Taking zvol snapshot: $SERVO_CI_ZFS_CLONE_PREFIX/$image_name@$snapshot_name"
+zfs snapshot "$SERVO_CI_ZFS_CLONE_PREFIX/$image_name@$snapshot_name"
 
 >&2 echo '[*] Done!'
diff --git a/windows10/build-image.sh b/windows10/build-image.sh
index f44fe6a..2a6ab25 100755
--- a/windows10/build-image.sh
+++ b/windows10/build-image.sh
@@ -1,5 +1,5 @@
 #!/usr/bin/env zsh
-# usage: windows10/build-image.sh
+# usage: windows10/build-image.sh <snapshot_name>
 image_dir=${0:a:h}
 script_dir=${0:a:h}/..
 . "$script_dir/common.sh"
@@ -9,6 +9,7 @@ cache_dir=$script_dir/cache
 . "$script_dir/inject.sh"
 undo_commands=$(mktemp)
 image_name=servo-windows10
+snapshot_name=$1; shift
 
 >&2 echo '[*] Caching downloads'
 download "$cache_dir" https://fedorapeople.org/groups/virt/virtio-win/direct-downloads/archive-virtio/virtio-win-0.1.240-1/virtio-win-0.1.240.iso ebd48258668f7f78e026ed276c28a9d19d83e020ffa080ad69910dc86bbcbcc6
@@ -18,6 +19,7 @@ zfs list -Ho name "$SERVO_CI_ZFS_CLONE_PREFIX/$image_name" || zfs create -V 90G
 
 >&2 echo '[*] Creating libvirt guest (or recreating it with new config)'
 if virsh domstate -- "$image_name"; then
+    virsh destroy -- "$image_name" || :  # FIXME make this idempotent in a less noisy way?
     virsh undefine -- "$image_name"
 fi
 virsh define -- "$image_dir/guest.xml"
@@ -71,8 +73,7 @@ fi
 >&2 echo '[*] Checking that Servo was built correctly'
 ./mount-runner.sh "$image_name" sh -c 'ls -l init/built_servo_once_successfully'
 
-snapshot=$(date -u +\%FT\%RZ)
->&2 echo "[*] Taking zvol snapshot: $SERVO_CI_ZFS_CLONE_PREFIX/$image_name@build-image-$snapshot"
-zfs snapshot "$SERVO_CI_ZFS_CLONE_PREFIX/$image_name@build-image-$snapshot"
+>&2 echo "[*] Taking zvol snapshot: $SERVO_CI_ZFS_CLONE_PREFIX/$image_name@$snapshot_name"
+zfs snapshot "$SERVO_CI_ZFS_CLONE_PREFIX/$image_name@$snapshot_name"
 
 >&2 echo '[*] Done!'