The lagna moves while you wait
The rising sign moves about one degree every four minutes, so a sign is up for about two hours. Twenty minutes off is usually still the same lagna. If the birth falls near the change from one sign to the next, even a few minutes can be the wrong Vedic chart. That is why an exact birth time matters for Jyotish, more than for a sun-sign column.
If you do not know the time
Please do not guess. A guessed time can look precise and still be the wrong kundli. VedicAstrology.com reads the chart calculated from the time you give. If you have no record, wait until you do. A hospital note, a family paper, a register. That is kinder than inventing a clock.
Place of birth belongs with the time
Lagna also depends on where you were born. The same minute in two cities is not the same rising sign. Enter the place as it was then. Online Vedic astrology here uses that place with your date and time to draw the sidereal chart, then a reader answers from it in a familiar messaging format.