Dear,

Good point to notice , i honestly havent noticed it before ,

but as a resolution i might suggest couple of ideas ,

first is the one  u already said , u could change the data field into string , and when ever u want to compare it with another date , u could convert it first , if u perform the comparing inside the workflow process u could use functions to convert it then compare it , so is for forms comparing , or as i do believe that if u retrieve that string into calendar control inside ur form , it will convert it by itself into date then u could perform the compare u need .

second idea , is that the problem u r facing happens when u leave the calendar empty , then it sends null as a string , which doesnt suit ur process data field ,

would it solve ur issue if u create an expression , to check whether the calendar is empty or not ? and if its empty , just send a dummy date/time ?

maybe an expression like , if Calendar is empty , (01/01/1753) else , (return the calendar value) .

Hope it helps!

Regards

Ahmad Yaqub.