I'm not aware of one, but it sounded like a cool problem, so here's my whack at it (VB.NET):
Private Function ConvertDateTimeToStringRelativeToNow(ByVal d As DateTime) As String
Dim diff As TimeSpan = DateTime.Now().Subtract(d)
If diff.Duration.TotalMinutes < 1 Then Return "Now"
Dim str As String
If diff.Duration.TotalDays > 365 Then
str = CInt(diff.Duration.TotalDays / 365).ToString() & " years"
ElseIf diff.Duration.TotalDays > 30 Then
str = CInt(diff.TotalDays / 30).ToString() & " months"
ElseIf diff.Duration.TotalHours > 24 Then
str = CInt(diff.Duration.TotalHours / 24) & " days"
ElseIf diff.Duration.TotalMinutes > 60 Then
str = CInt(diff.Duration.TotalMinutes / 60) & " minutes"
Else
str = CInt(diff.Duration.TotalMinutes).ToString() & " minutes"
End If
If str.StartsWith("1") Then str = str.SubString(0, str.Length - 1)
If diff.TotalDays > 0 Then
str &= " ago"
Else
str &= " from now"
End If
Return str
End Function
It's really not as sophisticated as ones that already exist, but it works alright I guess. Could be a nice extension method.