<$action-setfield mynow=<<now YYYY0MM0DD>>/>
Default tidders is only to open tiddlers when you open the wiki.
Do you want this date time field set on each tiddler when it is opened?
Are you asking for an action on open tiddler?
I have learned a fair bit around this but want to clarify what you want before I comment in more detail.
There are tricks, limitations and work arounds.
Regards
Tony
<$list filter="[has[open-actions]!has[draft.of]]">
<$button>
<<currentTiddler>> {{!!open-actions}}<br>
Click me!
</$button>
</$list><$action-sendmessage $message="tm-notify" $param="SampleNotification"/><$action-setfield mynow=<<now YYYY0MM0DD>>/> <ul> <$list filter="[has[due]days:due[60]days:due[-60]!tag[done]!sort[due]]"> <li> <$link><$view field="title"/></$link> <<calc !!due - by:mynow!!mynow>> days <$list filter="[<currentTiddler>days:due[15]!days:due[2]]"> @@color:beige;^^upcoming^^@@</$list> <$list filter="[<currentTiddler>days:due[1]days:due[-1]]"> @@color:red;^^today^^@@</$list> <$list filter="[<currentTiddler>days:due[-7]!days:due[-2]]"> @@color:black;^^recent^^@@</$list> <$list filter="[<currentTiddler>days:due[-60]!days:due[-30]]"> @@color:yellow;^^over a month^^@@</$list> </li> </$list></ul>\define mynow()<$button><$action-setfield mynow=<<now YYYY0MM0DD>>/>set today</$button>\end<<mynow>>Perhaps you could explain why you want to do this?
I have a few algorithms up my sleave, eg
I create journal tiddlers every day, and extended it to include a journal-date field I can change if I want. I also have a field called item-touch-date I can time stamp on any tiddler I visit, dont modify but want to log I reviewed.
In a similar way If I add a operational-review date to a tiddler I list all tiddlers with this field before today. When I action each I time stamp them now, indicating the daily review is done and they are removed from the list, I have something similar for weekly and monthly etc...
With these methods created and modified continue to mean what they do because the other field time stamps are set to not update the modified date.
Regards
Tony