To understand dates appearing on certain grants you will need to know something about how the calender has changed over the centuries. Before the year 1752, dates were recorded according to the Julian Calendar. In 1752, Britain finally adopted the Gregorian Calendar (you can read the text of the British Calendar Act), which had been designed in 1582 to correct the errors of the Julian Calender. That year, ten days were removed from the calendar. Although the opposite of what you might think, this had the effect of adding ten days to any date. For example 15 November became 25 November. Moreover, the New Year which had started on 25th of March was set to begin on 1 January. (Actually, I am unsure of this. See When did different countries accept the modern calendar?)
The dates of grants (and patents, if any) have been transcribed as they appear in the document. In some instances dates are given with a slash to indicate the change of years. This will affect the way you enter a date for search.
Dates are entered as plain text. They must be in the form of Day-Month-Year. If you are looking for a date showing the change of years, add a slash "/" and the last digit of the following corrected year: 1728/9.
It is important to be aware of this change because surveys were completed before grants were issued, not after, as some dates suggest.
For example: John Warner's survey made for Rober Carter's grant (NN C:39) is dated 4 December 1729. However, the grant is dated 2 March 1729 giving the appearance the grant came before the survey. In reality the grant was issued on 2 March 1730 because of the 25 March start of the new year (New Year Day).
Be careful when you see dates between 1 January and 25 March before 1752.
I'm not an expert in Julian-Gregorian calendar conversion, so you may want to consult other online calendar resources for a better explanation of how it works, the history behind the calendar switch, calculators, etc.