Document /spend quick action date argument

doc/user/project/quick_actions.md

@@ -68,7 +68,7 @@ threads. Some quick actions might not be available to all subscription tiers.
 | `/due <date>`                         | **{check-circle}** Yes | **{dotted-circle}** No | **{dotted-circle}** No | Set due date. Examples of valid `<date>` include `in 2 days`, `this Friday` and `December 31st`.                                 |
 | `/duplicate <#issue>`                 | **{check-circle}** Yes | **{dotted-circle}** No | **{dotted-circle}** No | Close this issue and mark as a duplicate of another issue. **(FREE)** Also, mark both as related. |
 | `/epic <epic>`                        | **{check-circle}** Yes | **{dotted-circle}** No | **{dotted-circle}** No | Add to epic `<epic>`. The `<epic>` value should be in the format of `&epic`, `group&epic`, or a URL to an epic. |
-| `/estimate <<W>w <DD>d <hh>h <mm>m>`  | **{check-circle}** Yes | **{check-circle}** Yes | **{dotted-circle}** No | Set time estimate. For example, `/estimate 1w 3d 2h 14m`. |
+| `/estimate <time>`                    | **{check-circle}** Yes | **{check-circle}** Yes | **{dotted-circle}** No | Set time estimate. For example, `/estimate 1mo 2w 3d 4h 5m`. Learn more about [time tracking](time_tracking.md). |
 | `/invite_email email1 email2`         | **{check-circle}** Yes | **{dotted-circle}** No | **{dotted-circle}** No | Add up to six email participants. This action is behind feature flag `issue_email_participants`. |
 | `/iteration *iteration:"iteration name"`     | **{check-circle}** Yes | **{dotted-circle}** No | **{dotted-circle}** No | Set iteration. For example, to set the `Late in July` iteration: `/iteration *iteration:"Late in July"` ([introduced in GitLab 13.1](https://gitlab.com/gitlab-org/gitlab/-/issues/196795)). |
 | `/label ~label1 ~label2`              | **{check-circle}** Yes | **{check-circle}** Yes | **{check-circle}** Yes | Add one or more labels. Label names can also start without a tilde (`~`), but mixed syntax is not supported. |
@@ -95,8 +95,7 @@ threads. Some quick actions might not be available to all subscription tiers.
 | `/remove_zoom`                        | **{check-circle}** Yes | **{dotted-circle}** No | **{dotted-circle}** No | Remove Zoom meeting from this issue ([introduced in GitLab 12.4](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/16609)). |
 | `/reopen`                             | **{check-circle}** Yes | **{check-circle}** Yes | **{check-circle}** Yes | Reopen. |
 | `/shrug <comment>`                    | **{check-circle}** Yes | **{check-circle}** Yes | **{check-circle}** Yes | Append the comment with `¯\＿(ツ)＿/¯`. |
-| `/spend <time(-<h>h <mm>m)> <date(<YYYY-MM-DD>)>` | **{check-circle}** Yes | **{check-circle}** Yes | **{dotted-circle}** No | Subtract spent time. Optionally, specify the date that time was spent on. For example, `/spend time(-1h 30m)` or `/spend time(-1h 30m) date(2018-08-26)`. |
-| `/spend <time(<h>h <mm>m)> <date(<YYYY-MM-DD>)>` | **{check-circle}** Yes | **{check-circle}** Yes | **{dotted-circle}** No | Add spent time. Optionally, specify the date that time was spent on. For example, `/spend time(1h 30m)` or `/spend time(1h 30m) date(2018-08-26)`. |
+| `/spend <time> [<date>]` | **{check-circle}** Yes | **{check-circle}** Yes | **{dotted-circle}** No | Add or subtract spent time. Optionally, specify the date that time was spent on. For example, `/spend 1mo 2w 3d 4h 5m 2018-08-26` or `/spend -1h 30m`. Learn more about [time tracking](time_tracking.md). |
 | `/submit_review`                      | **{dotted-circle}** No | **{check-circle}** Yes | **{dotted-circle}** No | Submit a pending review ([introduced in GitLab 12.7](https://gitlab.com/gitlab-org/gitlab/-/issues/8041)). |
 | `/subscribe`                          | **{check-circle}** Yes | **{check-circle}** Yes | **{check-circle}** Yes | Subscribe to notifications. |
 | `/tableflip <comment>`                | **{check-circle}** Yes | **{check-circle}** Yes | **{check-circle}** Yes | Append the comment with `(╯°□°)╯︵ ┻━┻`. |

doc/user/project/time_tracking.md

@@ -44,9 +44,9 @@ with [Reporter and higher permission levels](../permissions.md).
 ### Estimates
 
 To enter an estimate, write `/estimate`, followed by the time. For example, if
-you need to enter an estimate of 3 days, 5 hours and 10 minutes, you would write
-`/estimate 3d 5h 10m`. Time units that we support are listed at the bottom of
-this help page.
+you need to enter an estimate of 1 month, 2 weeks, 3 days, 4 hours and 5 minutes,
+write `/estimate 1mo 2w 3d 4h 5m`.
+Check the [time units you can use](#configuration).
 
 Every time you enter a new time estimate, any previous time estimates are
 overridden by this new value. There should only be one valid estimate in an
@@ -56,7 +56,9 @@ To remove an estimation entirely, use `/remove_estimate`.
 
 ### Time spent
 
-To enter a time spent, use `/spend 3d 5h 10m`.
+To enter time spent, write `/spend`, followed by the time. For example, if you need 
+to log 1 month, 2 weeks, 3 days, 4 hours and 5 minutes, you would write `/spend 1mo 2w 3d 4h 5m`.
+Time units that we support are listed at the bottom of this help page.
 
 Every new time spent entry is added to the current total time spent for the
 issue or the merge request.
@@ -66,6 +68,11 @@ days from the total time spent. You can't go below 0 minutes of time spent,
 so GitLab automatically resets the time spent if you remove a larger amount
 of time compared to the time that was entered already.
 
+You can log time in the past by providing a date after the time. 
+For example, if you want to log 1 hour of time spent on the 31 January 2021,
+you would write `/spend 1h 2021-01-31`. If you supply a date in the future, the
+command fails and no time is logged.
+
 To remove all the time spent at once, use `/remove_time_spent`.
 
 ## Configuration