To make a date in this field a required value, but only if status = "online", go to manage | database | fields, find the definition for this date field and double click it. Set up on of these validation calculation:
IsEmpty ( status ) or ( Status = "online" and Not IsEmpty ( self ) ) // allows status field to be empty.
( Status = "offline" ) or ( Status = "online" and Not IsEmpty ( self ) ) // Requires that status be either "online" or "offline".
How best to dynamically control the visibility of this date field will depend on the version of FileMaker that you are using. If you are using FileMaker 13, there's a new "hide when" setting that can be used to hide the field when status does not equal "online".
There are also ways to do this in older versions. If you are not using FileMaker 13, let me know and I'll share a demo file of methods for controlling visibility when using an older version of FileMaker.
Thank you very much for your answer. I am using filemaker 12 now. But if u could send a file like what u explained above(including the online offline stuffs+invisible field), then that will help me more to understand.
Once again thank u very much Philmodjunk
Here's the file that I was describing. I have not altered it to make any of the examples specific to this thread: https://www.dropbox.com/s/8levaz6deiyzjr8/LayoutObjectVisibilityControlDemo.fp7
You can use Open from FileMaker's File menu to open this file in order to get a copy converted to the .fmp12 file format.