TimeSums adds and subtracts time. It has three clocks A, B and C. Clock A can be set to any time value within a three day period, yesterday, today or tomorrow. Clock B can be set to any value between minus 23 hours 59 minutes, through zero hours : zero minutes, to plus 71 hours 59 minutes. The result of the subtraction, (or addition), between Clock A and Clock B is shown on Clock C in either Am/Pm or 24 hour format. If the result is Out of Range it is indicated.Clocks A, B and C can be locked, once set, so that a variety of addition or subtraction problems can be solved. For instance: if you know a departure time and a journey time, with no locks locked, Clock C will show the result of Clock A plus Clock B. If however you know the departure time and have decided on an ETA, entering the departure time on Clock A and the ETA on Clock C, will show the journey time on Clock B. If you lock Clock C at the ETA and then alter the departure time, the new journey time will show on Clock B. If you alter Clock B, (journey time), Clock A will show the new time of departure required.The TimeSums user interface is designed to be easily used by the visually impaired. Clocks A, B and C can be displayed large size on iPhone, iPad or iPad Mini by swiping down on the leftmost clock numeral. Individual clock numerals also can be displayed large by swiping down on the numeral, incremented by tapping and heard audibly by speech synthesis, or just by keyboard clicks, (on iPhone and iPad), or silently. Swipe up to cancel large numerals display and to hear the clock or numeral time spoken, if Sound is on.TimeSums is specially designed for use in a marine context. Mariners do frequent time calculations when working out tides, time offsets, primary and secondary port tide time calculations and estimated times of arrival. TimeSums has a night vision preservation mode, when the display is shown in subdued red light. White light can impair night vision for up to 15 minutes after exposure to it so even looking briefly at a bright iPhone display can adversely affect a watch keepers night vision. Not so with TimeSums.Clock Numerals are incremented by a tap. They can be also decremented directly by a tap on the minus icon below the numeral. Incrementing past a numerals maximum returns it to zero. Decrementing past zero returns it to its maximum, (9, 5, 3 or 2 as the case may be). This means any clock number value can be entered with an absolute minimum of taps.Clocks A and B can be zeroed separately. Clock C zero is an all clocks, all locks and time now, reset. Time offsets of GMT + 1, 2 and 3 hours are provided to work out time zone differences, such as those encountered across national borders.The actual time now can be automatically shown on Clock A and includes any daylight saving time appropriately. Though shown as GMT time, the actual time will be the default timezone set in your iPhone or iPad settings utility.