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Date Time · Formula sheet

Date Time formulas

One printable sheet for 18 Date Time tools — formulas collected from each calculator’s How it works.

Date Time calculators →

24-Hours Clock Calculator

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\[ h_{24} = (h_{12} \bmod 12) + 12\cdot[\mathrm{PM}] \]

Map the 12-hour hour and AM/PM flag onto 0–23.

Age Calculator

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\[ \mathrm{age} = t_{\mathrm{as\text{-}of}} - t_{\mathrm{birth}} \]

Count full years, then remaining months and days.

Age On Date Calculator

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\[ age=(Y,M,D) \]

Count whole years, then remaining months and days.

Date Calculator

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\[ t' = t_0 + n\cdot 1\,\mathrm{day} \]

Add the day offset to the start date.

Days Ago Calculator

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\[ t = t_{\mathrm{today}} - N \]

Result date = today − N days.

Days From Today Calculator

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\[ t = t_{\mathrm{today}} + N \]

Result date = today + N days.

Days Until Calculator

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\[ \Delta = t_{\mathrm{target}} - t_{\mathrm{today}} \]

Δdays = target − today (calendar midnights).

Hours Ago Calculator

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\[ t = t_{\mathrm{now}} - N\cdot 1\,\mathrm{h} \]

Result = now − N hours.

Hours Calculator

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\[ \Delta h = (t_2 - t_1)/3600\,\mathrm{s} \]

Δhours = (t₂ − t₁) / 1 hour.

Hours From Now Calculator

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\[ t = t_{\mathrm{now}} + N\cdot 1\,\mathrm{h} \]

Result = now + N hours.

How Long Until Calculator

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\[ \Delta = t_{\mathrm{target}} - t_{\mathrm{now}} \]

Δ = target − now; express as d/h/m components.

Minutes Ago Calculator

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\[ t = t_{\mathrm{now}} - N\cdot 1\,\mathrm{min} \]

Result = now − N minutes.

Minutes From Now Calculator

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\[ t = t_{\mathrm{now}} + N\cdot 1\,\mathrm{min} \]

Result = now + N minutes.

Time Calculator

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\[ t' = t_0 + h\cdot 1\,\mathrm{h} + m\cdot 1\,\mathrm{min} \]

t′ = t₀ + hours + minutes.

Timezone Offset Calculator

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\[ t_{local}=(t_{UTC}+\Delta)\bmod 24 \]

Add the offset, then reduce modulo 24 into [0, 24).

Weeks Ago Calculator

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\[ t = t_{\mathrm{today}} - 7N \]

Result = today − 7N days.

Weeks From Now Calculator

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\[ t = t_{\mathrm{today}} + 7N \]

Result = today + 7N days.

Work Hours Calculator

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\[ H=t_{end}-t_{start}-b \]

Parse start and end clock times, subtract break, then add extra full days.

Keep going

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Date Time calculators ← All formula sheets