Bloom time refers to the time of season a perennial is expected to bloom on average. There are some perennials that are ever-blooming but the majority of perennials bloom at a specific time of the season. This is important to ensure a garden has color all season long. A garden with only spring blooming perennials won't look very attractive in mid summer and so on.
Working with the list I provided on the perennial page, I will provide the approximate bloom times for several varieties.
HIBISCUS - They come in a good variety of colors and two major sizes. There is a tall form that can get up to approximately 7' tall and a much lower shrub form. Typically they are late bloomers, starting anywhere from late July to early August and they will bloom till first frost.
LARKSPUR - They are mid to late season bloomers. Starting in mid to late July and blooming for about a three to four week period.
CORABELL - Corabell are not known for their bloom but rather for their attractive foliage. I have seen them bloom anywhere from mid June to mid July depending on the weather and variety. There bloom lasts about 3 weeks.
DAY LILY - These flowers depend on what variety is selected. There are several that are ever-blooming, meaning they can bloom all summer. Most have a one time bloom and when that occurs and how long it lasts, depends on variety and maturity. The ever-blooming usually bloom in mid June and again in late July. The one time bloomers usually start in late June and if mature can bloom through August.
SEDUM - Sedum are a late season bloomer usually opening in mid to late August. Some creeping varieties will bloom much earlier although their bloom is hardly noticeable.
BALLOON FLOWER - They usually start in mid to late July and depending on how mature can bloom through August.
BELL FLOWER - Bell flowers are related to the Balloon flower so the bloom times are very similar.
DIANTHUS - Dianthus also depends on which perennial variety is selected. There are some that are very early bloomers, early to mid June. There are also other varieties that bloom in mid July lasting to early August.
CANNA LILY - Another mid to late bloomer depending on amount of sun.
BLACK-EYED SUSAN - Mid to late summer with a few week bloom time.
CONE FLOWER - Very similar to the above flower.
YARROW - Again a mid to late summer bloomer with a few week bloom time.
GRASS PLANTS - Not known for bloom but they do plume anywhere from mid summer to fall depending on variety.
HOSTA - Not none for their bloom which occurs in early to mid summer, June.
CARNATION - Usually an early season bloomer around mid to late June lasting a short period of time.
DAISES - Mid season from early to late July depending on weather conditions.
GARDENIA - Early to mid June lasting only a couple of weeks.
MUM - Late season, September until frost.
GERANIUM - Mid summer from late July for a month or more depending on the maturity of the plant.
BEE BALM - Usually early to mid July lasting a couple of weeks.
ASTER - Very much like daises.
MALLOW - Late July to early August lasting for several weeks depending on maturity.
PINCUSHION - Early July lasting throughout the summer with proper care.
ANEMONE - Early season around early June to early July before the temperatures increase.
COLUMBINE - Similar to Anemone.
ASTILBE - Like to two previous plants but the blooms last longer.
VIOLA - Beginning in early June lasting throughout the summer.
BLEEDING HEART - Mid June lasting a few weeks.
There are many variables with regards to bloom times but the above referenced is an approximate guide to get started. Variables include things like sun, temperatures, previous winter condition, moisture, location and maturity of the plant.
The above lists and references provide enough species to have a garden with color all season long. I always suggest a continued effort to find more types of perennials that can further enhance the garden. A lot of what works is just trial and error.