How to fix [today] and [now] in a list

Question

I have the following issue: we use a SP list to send information through the steps of the process. On each step somebody adds information into the list, next person receives it, adds his own part of information and it goes to the next step and so on. We need to see the date and time when each step was done. For first and last step we see it by columns "created" and "modified" for intermediate steps we created the calculated column with following formula:

=IF([Column1]="Yes";TEXT(TODAY();"dd/mm/yyyy")&" "&TEXT(NOW.TIME();"h:mm");"")

Problem is that with every new information added into the row the date&time are updated. How can I fix the date&time that it keeps only the date&time of the moment when somebody added the information in the exact column which I placed in the formula. Maybe it is possible with SP designer?

I found the following answer on a similar question for workflow but don't understand how I can apply it for a List: SharePoint Designer Workflow, set field

Thank you in advance!

EDIT

Workflow try when a person chooses "Yes" in the Status column that in column "Date & Time" appears current date and time and that this time would not be changed if somebody updates the information in the string.

EDIT 2

EDIT 3

Finally it works this way, with the additional settings in "Date & Time" - default value "01/01/2014 0:00:00"

EDIT 4

Solution

"Date & Time" is the text fild with default value 1

Answer 1

It sounds like you shouldn't use a calculated column at all:

It would probably be easiest to use the workflow to set the field value when that intermediate step is complete as is sort-of suggested in your link; to get it working in your situation you need to make the column you're using as a calculated one now a date or text type, then make a workflow or add a new step to a workflow that occurs whenever an item is modified to check for your [Column1]="Yes" then perform the set a field's value action to the current date/time.

Alternatively, you could make whatever form the user in the intermediate step is using set that date value directly when they submit their information.

EDIT: I almost forgot -- make sure you only allow the workflow to set the date for your new column when an item is edited and [Column1]="Yes" AND the column that holds the date it was approved is empty, otherwise the date will always be updated every time the item is edited after [Column1]="Yes"