From:http://www.javascriptkit.com/script/script2/tengcalendar.shtml
----------------------------------------------------
1) Date (MM-dd-yyyy) 
HTML code: <input id="demo1" type="text" size="25"><a href="javascript:NewCal('demo1','ddmmyyyy')"><img src="images/cal.gif" width="16" height="16" border="0" alt="Pick a date"></a>
2) Date & Time(12-Hour) (dd-MMM-yyyy hh:mm:ss)
HTML code: <input id="demo2" type="text" size="25"><a href="javascript:NewCal('demo2','ddmmmyyyy',true,12)"><img src="images/cal.gif" width="16" height="16" border="0" alt="Pick a date"></a>
3) Date & Time(24-Hour) (dd-MM-yyyy HH:mm:ss)
HTML code: <input id="demo3" type="text" size="25"><a href="javascript:NewCal('demo3','ddmmmyyyy',true,24)"><img src="images/cal.gif" width="16" height="16" border="0" alt="Pick a date"></a>
-----------------------------------------------
Directions
1. Download the following zip file, which contains the necessary files plus a demo.
2. Upload "DateTimePicker.js" and "cal.gif" contained within the above zip file to your web site.
3. In the <head> section of your webpage that will display the Date Picker, insert the below code:
<script language="javascript" type="text/javascript" src="datetimepicker.js">
//Date Time Picker script- by TengYong Ng of http://www.rainforestnet.com
//Script featured on JavaScript Kit (http://www.javascriptkit.com)
//For this script, visit http://www.javascriptkit.com
</script>
4. Create the desired textboxes that will use the Date Time Picker. A sample textbox looks like:
<input id="demo1" type="text" size="25"><a href="javascript:NewCal('demo1','ddmmyyyy')"><img src="images/cal.gif" width="16" height="16" border="0" alt="Pick a date"></a>
As seen above, enter a unique ID into each <input> tag (in this case, "demo1"), and inside function "NewCal()", pass this ID as its first parameter. Below describes NewCal() in detail:
NewCal([textbox id],[date format],[show time in calendar?],[time mode (12,24)?])
a. You must enclose textbox id and date format with single quotes (').
b. Don't insert date separator in between of date format such as dd-MM-yyyy.
c. Parameters for "NewCal" function:
- Textbox id (Mandatory)
- Date format (optional) - If you don't specify date format, the default date format will be "dd-MM-yyyy". Other valid date formats for My Date Time Picker are: ddMMyyyy,MMddyyyy,ddMMMyyyy,MMMddyyyy.
- Show time in calendar? (optional) [true or false] - Specify "true" if you would like to let user to pick a time. Default value is "false".
- Time Mode (optional) [12 or 24] - Time shown in 12-hour or 24-hour format. Default is 24-hour format.