Dear all,
I have 5 subjects who had 5 visits at different dates, I want to create a new variable that has the number of days elapsed since the date of first visit of each subject

Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input str11 usubjid float visdat byte visnam1
"494" 21571 1
"494" 21599 2
"494" 21677 3
"494" 21851 4
"494" 22097 5
"787" 21579 1
"787" 21606 2
"787" 21690 3
"787" 21858 4
"787" 22103 5
"068" 21587 1
"068" 21615 2
"068" 21698 3
"068" 21868 4
"068" 22131 5
"246" 21594 1
"246" 21620 2
"246" 21704 3
"246" 21872 4
"246" 22167 5
"468" 21656 1
"468" 21679 2
"468" 21756 3
"468" 21931 4
"468" 22195 5
end
format %td visdat

for each line, I want to have a variable that has the number of days elapsed between the visit date at that line and the date when visnam is 1 for the subject
In other words for subject 494, the value of the new variable for the first observation will be : visdat - itself so it will be 0, but in the next line the new variable should be number of days between visdat at visit 2 - visdat at visit 1 , and so on.
so it always has to be days between date of visit at that line and the date of visit at visit number 1 for the subject.

I hope this makes sense and I hope I have made my self clear.

Many thanks for your time in advance.