From HoNwiki

Base Attack Time governs how many seconds a unit has to wait before it attacks, and how the unit responds to attack speed modifiers.

I really think this page should be changed to Attack Speed, like all other unit statistics such as movement speed. There is a lot of information that I've not included because it is not directly relevant to base attack time or because it is awkward to include, however a page called Attack Speed would have relevance with base attack time and could use this page inside it. Reminder: Proportionate AS DP

Basics

All units that can attack in the game are given a base attack time, similar to them having a base armor, base health, base movement speed where applicable. A unit's base attack time is given in seconds, and represents the delay before it can commence its next attack.

Base attack time can only be changed in heroes via skills at the moment, not items. Some percentage based slows and hastes can be seen to change base attack time in practice; a 50% attack speed slow can be seen as doubling a unit's base attack time, although technically not changing the base attack time at all.

Formula (someone make a picture, text looks ugly and hard to read)

A hero's attack interval can be shortened or lengthened by increasing or decreasing attack speed. In both Heroes of Newerth and Dota, the majority of these modifiers are not percentage based, so cannot be directly applied to a unit's attack speed. A unit has an unmodified attack speed of 1, which any increases or decreases in attack speed are added as if they were a percentage, that is, +80(%) attack speed would be +0.8. All attack speed modifiers must be put into the following formula.

To determine attacks per second:

(1+(Increased attack speed (IAS))/10)/Base attack time.

An imp that has been slowed by 50 (note: not 50%):

(1-(5/10))/1.25 = 0.4 attacks per second.

A Soul Reaper that has 25 agility, 45 attack speed and a 20% slow from Frostwolf's Skull:

[(1+(0.25+0.45)/10)/1.7] x 0.8 = 0.8 attacks per second.

Attack Speed required to increase attacks per second

Since HoN's servers run at 20 frames per second, a unit must reach the next 'step' in the server frame when attacking faster than once per second in order to receive an actual increase in damage output. This chart is made for all normal heroes, Magebane is an exception with a base attack time of 1.45.

Frames per attack	Seconds per attack	attacks per second	attack speed required
20	1.00	1.00	+70
19	0.95	1.05	+79
18	0.90	1.11	+89
17	0.85	1.18	+100
16	0.80	1.25	+113
15	0.75	1.33	+127
14	0.70	1.43	+143
13	0.65	1.54	+163
12	0.60	1.67	+183
11	0.55	1.82	+209
10	0.50	2.00	+240
09	0.45	2.22	+278
08	0.40	2.50	+325
07	0.35	2.85	+386
06	0.30	3.33	+467
05	0.25	4.00	+580
04	0.20	5.00	+750
03	0.15	6.66	+1033
02	0.10	10.0	+1600
01	0.05	20.0	+3300