The novel officially begins on April 4th 1984 (the reverse year of when Orwell wrote the novel which was 1948, thus he was writing about a future dystopia). The day is described as "a bright cold day in April, and the clocks were striking thirteen" with a "vile wind" blowing (p.3). This time of season and weather condition adds to the bleakness of the mood being created by Orwell as the novel opens. The clocks striking an hour which does not exist in a regular striking clock pattern is also significant as it foreshadows a society which has made major structural changes to basic institutions to suit its own purposes.
We know the date is April 4th because Winston has plucked up the courage to start writing a diary, which is a highly illegal act in itself attracting harsh punishments. It is significant that Winston, even though a government employee, cannot be sure of the exact date. He is only guessing based on his age of which he appears to be not entirely certain. This helps the reader to form a picture of a society where the citizenry are shielded from even knowing the most basic information related to exact time, suggesting it must be one of many measures to keep them ill informed for the sake of maintaining control.