Here is a simple calculator written by Ven Yuttadhammo for working out the correct times for monastic dawnrise and midday in the Theravāda tradition. Please read the About at the bottom of the page for more information on how the calculations are made, and what are their limits.
Some of the rules affected by this calculation: rules for lay people observing the 8-fold virtuous practices (sīla) on the uposatha; the rules about eating at the proper time affects both lay and monastic; for the monastics such things as the nissaya rules, maintenance of the rains-retreat, being together with the three robes rules and probation are all affected by this timing.