Scheduling Events With OnTime And Windows Timers
This page describes the Application.OnTime method and Windows
You may have the need to run some code periodically and
automatically, such as code to pull data from an external data store. With some
simple VBA code, you can call upon the the Application's OnTime method to
automatically run a procedure. This pages describes how to do this. It also
includes a section on using the timers provided via the Windows API functions.
As parameters, the OnTime method takes a specific data and time
at which it should run the procedure and the name of the procedure to run. It is
important to remember that you provide the date and time to run the procedure,
not an offset from the current time. If you need to cancel an OnTime, you must
provide the exact time that the event was schedule to take place. There
is no way to tell Excel to cancel the next OnTime event or to cancel all the
pending OnTime events. Therefore, you need to store the time at which the
procedure is to run in a Public variable and use that variable's value in calls
Note that if the workbook containing the procedure to be executed by OnTime is closed before the procedure is run,
Excel will open the workbook before running the procedure and will leave the workbook open after the procedure is complete.
For example, declare Public variables in a standard code module,
outside of and before any procedure (Sub or Function) declaration:
Public RunWhen As Double
Public Const cRunIntervalSeconds = 120
Public Const cRunWhat = "TheSub"
To start a repeatable timer, create a procedure named StartTimer as shown below:
RunWhen = Now + TimeSerial(0,0,cRunIntervalSeconds)
Application.OnTime EarliestTime:=RunWhen, Procedure:=cRunWhat, _
This stores the time to run the procedure in the variable
RunWhen, two minutes after the
Next, you need to write the procedure that will be called by
OnTime. For example,
This procedure executes whatever code you include in it, and
then at the end calls the StartTimer procedure to
schedule another OnTime event. This is how the periodic calls are implemented.
Note that if you close the workbook while an OnTime event is pending, Excel will
re-open that workbook to execute the procedure and will not close the workbook
after the OnTime event is finished.
At some point, you or your code will need to terminate the
OnTime schedule loop. To cancel a pending OnTime event, you must provide the
exact time that it is scheduled to run. That is the reason we stored the
time in the Public variable RunWhen. You can think of the RunWhen
value as a unique key into the OnTime settings. (It is certainly possible to have multiple
OnTime events pending. In this, you should store each procedure's scheduled time
in a separate variable. Each OnTime event needs its own RunWhen value.)
The code below illustrates how to stop a pending OnTime event.
On Error Resume Next
Application.OnTime EarliestTime:=RunWhen,Procedure:=cRunWhat, _
In addition to Excel's OnTime method, you can use the Windows
Timer functions provided via Window API. Windows Timers are automatically
rescheduled and will continue to "pop" until you terminate the timer with the
KillTimer API function. With a Windows Timer, you provide the interval, in
milliseconds, that the timer will "pop". The timer will "pop" at that interval
until terminated with KillTimer.
These procedures require Excel 2000 or later, since we use the
AddressOf operator. The code will not work in Excel 97 or earlier versions.
|A NOTE OF CAUTION: If the code executed by the timer changes a
cell value, and you are presently in edit mode in Excel (e.g., entering data in a
cell), Excel will likely crash completely and you will lose all unsaved work.
Use Windows timers with caution.
To use Windows timers, paste the following code into a standard
Public Declare Function SetTimer Lib "user32" ( _
ByVal HWnd As Long, _
ByVal nIDEvent As Long, _
ByVal uElapse As Long, _
ByVal lpTimerFunc As Long) As Long
Public Declare Function KillTimer Lib "user32" ( _
ByVal HWnd As Long, _
ByVal nIDEvent As Long) As Long
Public TimerID As Long
Public TimerSeconds As Single
TimerSeconds = 1
TimerID = SetTimer(0&, 0&, TimerSeconds * 1000&, AddressOf TimerProc)
On Error Resume Next
KillTimer 0&, TimerID
Sub TimerProc(ByVal HWnd As Long, ByVal uMsg As Long, _
ByVal nIDEvent As Long, ByVal dwTimer As Long)
Run the procedure StartTimer to begin the periodic
timer. The variable TimerSeconds indicates how often,
in seconds, the timer is to "pop". The SetTimer
function takes a value in milliseconds, so the code multiplies
TimerSeconds by 1000.When the timer "pops" Windows
will call the procedrue TimerProc. You may name this
procedure anything you want, but it must be declared with the
parameters exactly as shown above. If the parameters differ from what is shown
above, Excel will crash. When Windows calls TimerProc,
it passes the following parameters:
|This is the HWnd (Windows Handle) of the Excel application. You can
ignore this parameter.
|The message ID of a timer message, value of 275. You can ignore this
|This value indicates which timer is being "pop" if you have more
than one Windows timer in effict. This is the value that was returned
from the SetTimer API function.
|The number of milliseconds that Windows has been running. This is
the same value that you would get from the
GetTickCount Windows API function.
To terminate a Windows timer, use the EndTimer
procedure shown above, which calls KillTimer to
actually terminate the timer.
This page last modified: 23-Oct-2008.