To find a relative date 90 days in the past from a specific date, such as 14th February, you can manually calculate it by subtracting 90 days from the given date. In this case, you would subtract 90 days from 14th February to determine the date that falls 90 days earlier. One way to do this is by counting back 90 days on a calendar.

Another method to manually calculate the relative date 90 days in the past is to convert the days into months and days. Since a month typically has 30 days, you can calculate that 90 days is equal to 3 months. Subtracting 3 months from 14th February would bring you to 14th November, which is 90 days before 14th February.

If you prefer to use a mathematical approach to calculate the relative date 90 days in the past, you can convert the given date into a numerical value. For instance, you can assign 14th February as day 45 (14 days from the start of February, added to 31 days in January), and then subtract 90 days to arrive at day 45-90 = day -45. Working backward from February, day -45 corresponds to 14th November.

By following any of these manual calculation methods, you can effectively determine the relative date that falls 90 days in the past from the given reference date, 14th February. These techniques can be applied to various date calculations and are a useful skill to possess for a range of scenarios requiring date manipulation.