Should I mail asking for my date of Joining?

Iam a fresher and I got selected in a company in January in campus placements. I got an offer letter in March which I signed and sent them. Later they have sent a mail that they would inform the Date of joining on 13th of this month. But I didnot receive any further mails. After two days when we called and asked the HR, they said that they have not yet decided the date and they would inform as soon as they do. It has been two weeks since that and Iam thinking whether I could mail them asking for the date.

Should I do so? If so, how should the mail be? Can someone give me a sample letter?

Note: Later only one guy from the batch of students who got selected during the process got the joining date recently.

I will be very thankful for your answers.